I just went through a little serpentine belt drama with my 1995 Safari, 178k at the time. I had some noise coming from the belt, and immediately found the tensioner pulley as the culprit. I had my mechanic replace it, and just for good measure put a new belt on it because it looked like it still had the original and was the cut type, not the constant rubber ribs with grooves.
A day later, the belt starts a chirping noise. The noise doesn't really change much no matter the condition, just speeds up with the engine rpm. My mechanic and I decided to try changing the tensioner thinking maybe there was some length difference in the different type of belt. It would come with a new pulley, so we decided to move the new pulley I had already paid for to the idler position and take that variable out of the equation. For a few minutes, I was happy. Then the same noise returned.
So, we started thinking it had to be the brand new belt. The original belt had no issue, but the bad pulley. This made us conclude there was no alignment issue. I had changed the tensioner and that pulley, so it suggested there was no tension issue.
I had purchased the new belt from O'Reilly auto parts, because they had it in stock. We noticed that the back side of the belt wasn't very smooth. There were tiny peaks and valleys in the stack of strands. We also noticed that the inside of the belt had a shine to the rubber in the valleys of the grooves. After talking with the manager of the O'Reilly store, they said they would warranty the belt for exchange. When the new one came, it had the same issue. We called the manager back, and were offered a Gates belt. If I had a choice I would have gone with it from the beginning, but it wasn't in stock at any of the local parts stores the time.
When we installed the Gates belt, we noticed it was quite a bit shorter. It was very smooth on the back side, and had no shiny surface in the grooves. It was made in Mexico like the first belt, but obviously under tighter controls. As soon as it was installed, not a peep. I have since driven over a thousand miles and it's still nice and quiet.
If you are having an issue, take a good look at your brand new belt. I could have probably avoided buying a new tensioner trying to solve the issue, but it's not too hard to take knowing that the parts that were replaced were probably going to fail soon enough as they looked original. Better to have new parts on there than to risk old ones failing 3000 miles from home where the mechanic isn't also a friend.
Hope this helps