1996 olds 88 unusual noise

I just replaced the plastic plenum, flushed cooling system and changed oil on 96 olds 3800 seriesII engine. Engine seems to run fine except for a noise that sounds like a belt squeal. I removed the belt and run the engine for about minute, the noise was still there. I installed the belt and took the car for a test drive, at 60 mph the noise almost disappears, but when I let up on the gas, it gets louder.

You either forgot to put the PCV valve in or you forgot to install the rubber o-ring seal for the PCV valve. Essentially you are putting the entire engine under vacuum and the sound is likely the valve cover gaskets sucking in and resonating. The PCV valve gets installed under the MAP sensor on the passenger side of the plenum.

I just replaced the plastic plenum, flushed cooling system and changed oil on 96 olds 3800 seriesII engine. Engine seems to run fine except for a noise that sounds like a belt squeal. I removed the belt and run the engine for about minute, the noise was still there. I installed the belt and took the car for a test drive, at 60 mph the noise almost disappears, but when I let up on the gas, it gets louder. Thanks, you are right, I guess I got in a hurry and forgot the pcv valve. I put the valve in and the car sounds good. Thanks again.

My pleasure!