The best way I can describe the noise is that it sounds like a steel lawnmower throtttle cable vibrating up against a piece of metal or plastic.

The noise occurs only at a particular rpm just a little bit above idle. More noticeable in gear at that rpm, but does occur once in awhile in park if I touch the accelerator pedal a few times. It occurs mainly in the morning when I am slowing down for a stop sign or red light, and also again when I am accelerating to leave the intersection.

More pronounced when truck is cold.

It does not matter what state the ventilation system is in: Totally off or max A.C. the problem is the same.

It was a problem when the truck was brand new. It was one of the punchlist items they fixed on my first trip back to the dealer for service. They did not tell me what the problem was, but since they seemed to fix it with little difficulty, I'm thinking they were familiar with this problem like it may have been a common problem on these vehicles from the factory.

It stayed away for 12 years, but this summer I had the A.C. compressor changed and the noise was back again when I picked the vehicle up. Compressor installer claims ignorance, and say they had nothing to do with the problem.

I have tried isolating the noise by listening with a rubber hose, but have had no luck. I disasembled the dash as much as I feel comfortable doing without making it a major project. One of the problems is that I've been trying to do this by myself because help is not readily availabe, but to diagnose it correctly I should have one guy goosing the accelerator with the vehicle in drive, with the parking brake on while I listen around with the hose, but that has not happened yet.

The vehicle has been ok all in all. It is a 96 S-10 with a 4.3 4wd, 4 spd auto with LE trim package. 155k miles.

The problem was the accelerator cable was resting on the pcv valve. Vibrations were resonating through the cable and were causing the vibration noises at certain rpms. When the guy changed the a.c. compressor he disconnected a plastic tie ring that was previously used to keep the accel. cable up high enough from the pcv to keep it from happening. I think he did it to access some wiring to the compressor. Anyway, listening through the rubber hose while sitting in traffic told me the area of the noise was where the accelerator cable came through the firewall. I popped the hood and checked along the cable and heard the noise of the pcv valve. I reconnected the plastic tie ring to raise the cable and voila! - eight hours into the project - problem solved. I've spent eight hours of my life on many less useful projects in the past so I guess I'm happy. I told the a.c. compressor installer about it. He said "I'll have to remember that one". I hope he does.