Astro power steering hiss noise from steering wheel

[RegalCustom]

I have a 2002 Astro LT 4.3L rwd, and after changing the steering gear box it makes a hissing sound from the steering wheel through the steering colum. It sounds like the power steering fluid that is being pumped through the steering gear box. I have replaced the power steering gear twice and it still makes the hiss noise. Also, flushed the system and replaced with GM PS fluid and also tried a synthetic fluid. No change. The sound occurs when the wheel is turned in either direction at idle or while moving. I have read similar threads about this type of noise with other GM trucks but can't seam to find a solution.

Thank you for any possible suggestions.

[redfury]

Possible air in the system? Bleeding is as simple as turning the wheel to the left and the right, but not to the stopping point, just before a few times, then returning to center and letting the engine run a few more minutes to help push the air up and out, checking the fluid as you go along.

If this doesn't change the noise, then it may be the pump itself? I'm assuming when you talk the gearbox, you are talking the gearbox at the end of the steering shaft, correct? Because if that's the case, the only thing it should be is something in the hydraulics. A whine or noise from the pump could be transmitted through the column, as well as anything else in the suspension that could be making noises. A hiss though tends to tell me something that is constant.