Hi, hoping maybe someone can help me out here. I have a 90 Mighty Max with replaced engine w/65000 miles. I'm experiencing a high pitch sound when I accelerate. If I very gingerly press on the accelerator, it doesn't do it. If I accelerate a little more aggressively, it will do it until the engine isn't under load. It's very similar to a belt slipping - when the belt catches up with the engine, the squeal stops. This is kind of similar. It seems to do it, like I said, a little more under torque.

I know it's not a belt. It sounds like it's coming from the engine area somewhere, but I can't put my foot on it because it barely does it if I'm not driving. If not driving, and I "crack" the throttle body, I can hear a faint sound that's similar. However, most of the time it's most profound only when driving and only when the engine is under load. It kind of sounds like a vacuum leak, but I don't think it is. I've replaced all lines. Plus, it's so time specific that I don't think it's vacuum.

So, I've been thinking of possibilities. AC clutch? Would the AC clutch be able to do this? Fan clutch? I'm kind of grasping at straws here. The reason I've mentioned those two things is because they contain a "clutch", which would kind of make sense if it was slipping (making noise) and then the noise would stop when the item caught up with the engine rpm's. It's just an idea. Oh, and it does it hot or cold, but seems to do it a little worse when it's cold.

It kind of sounds like a turbo sound, although not exactly. It also kind of sounds like an intake sound - like maybe if one cut a 2" hole in the intake tube or something. You know, you would hear that sucking, whistling sound...

I'm just throwing this out there just in case someone might know off the top of their head. I suppose I could try holding the fan or something and maybe check the fan clutch that way (never did anything like that before). The AC clutch, I wouldn't know how to check, and I don't even know if the clutch is a factor if the AC isn't turned on (the truck is making this noise with the AC off - haven't tried it with the AC on...)

Any ideas would be appreciated. I'm nearly sure it's not anything drivetrain related, because it does it very specifically related to acceleration. If I "feather" the acceleration, and take off very slowly, it doesn't do it...

Oh, and it doesn't seem to be affecting performance, or how the engine runs...

Thanks in advance... <img src="/forums/images/graemlins/cheers.gif" alt="" />