I have a mostly stock '99 24v with 75,000 miles. When i am am going at a constant speed around 65 or faster my turbo makes a popping sucking sound but when the truck is under load while accelerating it does not make the sound. the only thing i have done is put a k/n, drilled holes in the air box, 4" turbo back ATS exhaust, and a 65 hp bd chip.

 

Have you checked the exhaust manifold bolts for proper torque?
They are very prone to back off and even fall out.
There is a TSB on this.

 

Could you possibly have a boost leak?

 

thanks, I will be sure to check for a boost leak and manifold bolt torque

 

You might want to take the turbo inlet hose off and look at the turbine shaft. Look for excessive play (up/down, in/out) and any deformity on the blades.<br><br>Mike