[Quarterwave]

Hey all,

Further to the subject, I've been trying to find out the source of a knocking noise coming from the rear LHS of my 96' Astro 4.3 cargo van (77, XXX miles).

As I don't have the space or tools to do this type of work, I have a local mechanic that has replaced :

- the rear shocks (which were shot)
- trans mount (bolt holding it in place was broken)
- both rear wheel bearings (left hand side was slightly leaking)
- changed out the diff fluid whilst inspecting the differential.
- checked spare tire to make sure it was seated correctly.
- replaced worn rubber shims in the rear suspension leafs
- check all suspension links and u-bolts for movement, incl. driveshaft coupling at the front of the rear diff.

The problem sounds like a muted knocking sound that comes from the rear left hand wheel, which increases in speed as I drive faster, and then gets slower as I slow down. It is most noticable in the 30-40 mph range, esp when I am coming off an interstate and are coasting (ie : foot not on the gas pedal) to either a stop or slow turn.

After getting the above work done (wheel bearings were this week), we took it for a test run, and sure enough, it's still there. So, we drove in reverse for a bit, and the problem was still there, likewise when we took it out of gear and into neutral @ around 20 mph (back street behind the shop) - still, the knocking is there.

The mechanic is stumped, as with only 77k miles on this truck, he said it shouldn't be bad - even though the oil in the diff was a little bit burnt.... He also said that the bearings on either side of the diff itself (you can see them with the diff cover off) looked ok.... but he didn't pull them to check (from the look of it, they were held in place by a kind of "C" shaped housing, that is bolted into the diff housing)....

After sinking hundreds of dollars into this thing, I'm at the point where I just want this done once and for all.....

So, does this sound like a problem with the diff itself ? As after looking at the gears, there were no metal bits in the housing, no shavings, and to my eye, they looked ok (granted, I'm not a mechanic so....).

If I do have to do the diff, are there any heavy duty diffs that I can put into this van (drop in / bolt in replacement), as I do not want to deal with this problem again (I drive mostly highway miles with moderately light loads - under 800 lbs) - and if so, where can I get them from - unfortunately, getting a full rear end from a salvage yard is not an option, as I have no way to move it from the salvage yard, nor the type of friends that will help me lift it.

Thanks guys for any information or help.

 

[astro355]

Did you check the tread of the tire on that side? Rock? Nail? Anything else stuck in the tire? Same for the drum.

 

[Quarterwave]

Yep, brand new tires all round (2 weeks old), and we used a lathe to re-surface the rear left hand wheel drum.

When we took it for a test run, the mechanic jumped over the middle seat and put his ear to the floor whilst I was driving - and said he could hear the noise coming from underneath the floor in the center of it. After we stopped he said that the sound might be traveling down the axle housing and into the left wheel area....

Not sure how accurate that is, as to my thinking, if that was the case, wouldn't it travel down both sides of the rear end ? Admittedly, he seems to be working with me and is now taking this one almost like it's a challenge, as last night we were back in the shop well after 7 p.m and he didn't charge me for the time so....

 

[astro355]

You can always rotate the tires and see where the noise comes from then.

 

[Quarterwave]

Thanks Astro,

I did that when I got the new tires.... I needed 1 new one (rear left) when I bought the van, and the tire place put the white letters on the inside, so when I went back a few weeks later to get the 3 others, I had them put that rear left tire on the other side so that the letters would face out, and then another new one (letters facing out) on the rear left wheel (if that all makes sense).... but, the noise is still coming from that rear left hand side.

In the event I need a diff - do you know what code I need to look for on the inside of the passenger door ? As there are a tonne of letter / number combinations on a sticker that is on the passenger door, and I imagine that they are a list of what the van has in and on it....

Also - does anyone recommend a good place to source a new diff and any other associated parts ? Advance Auto has them for around $200.... and I'm hoping that my mechanic will do a good deal on the labour to install it - i hope....

Thanks again for your interest and help.

 

[astro355]

You said that the LH side wheel bearing was leaking. Did any of that grease get on the brake shoes?

 

[Quarterwave]

None that I could see - the bearing was only leaking very slightly - a little bit wet @ the outer edge, with a small amount of dust / dirt collecting @ the bottom of it.

 

[Jasen]

Might try swapping the tires left to right. It's free and it would assure it's not a tires issue. Just because it's new, doesn't mean you didn't get a bad one.

 

['cudapaul]

Does the "knock" correspond to a position on the wheel; does the rate of the knock happen once each time the wheel makes a full rotation, or more, or less.
This would give some insight into what rotational component(s) could contribute to the problem.

Also, it may take more than a visual inspection of the diffrential to determine an internal problem; it could be a carrier bearing, one of the spider gears, the locking pin, or pinion bearings.

Could even be a bent axle or warped/dinged/out of round/bent brake drum.

You might want to locate a company that repairs differentials, they more have more specfic experience in finding the problem than a regular auto mechanic.

 

[interestingfellow]

It's not an easy difference to hear, but do you think the frequency of the knock is in line with the rotation of the tires or drive shaft? If it's a quicker knock, check the driveshaft, universal joint bearings, and the yolks that hold the spindle in the universal joint.

If it sounds like the frequency matched the rotation of the tires/axle, then I would check the inner bearings in the diff, and axle. You will also need to check everything around the hub/tire. Make sure nothing is loose and knocking the tire, and that the guts of your rear hub are all in place how they should be.

I once had lost a needle of a bearing when repacking said bearing, and it produced an almost steady knock. At a different time, the bearings in my drive shaft universal joint were rusted as hell, and it had lost needles/shred them up, and it also produced a steady knock. Both knocks were proportional to speed and/or rpms.

It *could* be in the transmission and transfering down the driveshaft to the rear end, but your mechanic should have noticed that if he was hunting around with his ear.

As far as the rear end goes, as long as it has the same ratio (which I believe is stamped on the outside of the diff housing) and it came from an astro in your astro's range (eg: I have a 90, so i can go from 85-94 for parts), then you can throw it in. Other rear ends might work, provided they have the correct ration, and wheel base (width). But that is at your discretion.

Taking the rear end out by yourself can be a bitch, but it's certainly manageable for the backyard novice (which I am). The first thing I do after acquiring a different vehicle, is to go get the hayne's (comprehensive, but simplified) or chilton's (comprehensive with anything you might want to know about what many people don't mess with) manual. $20 at advanced auto (or similar).... Go get one.

Pull the rear end out, and roll it to a location you can back your astro up to. Stand it up vertical (leave on the bottom tire/rim and take off the tire from the end that will go upwards), back up the van, set the high end in the van, and slide/grunt it into place. A movers blanket kicks *** in this scenario. Also, many times, a junkyard might send an employee to help you move the item, or use thier forklift to move it. Don't ask them to help you yank it out (they get annoyed), but only to move it by/into your van/trailer.

God bless spellcheck...

 

[Quarterwave]

Thanks for all of the ideas everyone....

Cudapaul - yes, it seems to be relatively in sync with the speed that I'm driving at - as in it knocks faster the faster I am driving - although it's only really noticable when the drivetrain is NOT under load and is often quiet for the first 1/2 mile or so after it has been left overnight.

When the mechanic and I went for a test drive last week, we drove and put the trans into neutral, and coasted for a bit.... the noise was still there, even though the transmission was disengaged.... we even put it in drive on the hoist, but @ idle speed, and we couldn't hear the noise - it seems to only come in from about 15 mph onwards.... and is most extreme in the 30-40mph range.

Although, if I'm doing say, 55 mph, I would guesstimate that the rpm would be in the 2000 range (no tach in my van to reference), and it definately does not sound like it is knocking @ 2000 times per minute.

Well noted on the suggestion to take it to a rear-end specialist...

Interestingfellow - thanks for the info. Unfortunately, I live in an apartment complex, and it just isn't practical to do a rear end swap or repair in my parking lot, although again, I appreciate the insight.... I actually did pick up the factory service manuals for the van just after I bought it - as I have needed to change a few things (A/C blower motor, front bumper etc....), and the manuals definately helped.

In regards to the frequency of the knock, you are right - it is very hard to tell. Although, to me, it seems more like it's coming from the wheel hub - which is why I had the rear wheel bearings replaced and the brakes all checked out.... although, I have to admit, I didn't check the axles to see if they were warped.... having said that, nothing looked out of place with them - as far as I can remember.

I just have a gut feeling that it's those inner bearings that are mounted either side of the differential, just near the outside of the diff housing.... but admittedly, can't really prove it.... so, as per my response to Cudapaul, I think I'm just going to have to take a day off work and go to a diff specialist and find out what's what.

The problem is that the easy (and most costly) fix is just to replace the whole lot, and that's what I find the mechanics in my area telling me (maybe it's the Australian accent that makes them think I have no clue - if only they knew I used to run a high 11 sec Charger in high school ....

Anyone know a good mechanic in North Miami ?