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="" />

bad bearing in the timing belt/waterpump

possible, but unlikely - I just exchanged the engine and prior to installing the 65K engine, I changed water pump and timing components. So, my guess would be highly unlikely, but not impossible, because it wouldn't be the first time new parts were defective...

Thanks...any other ideas? All are welcome...

timing belt to tight

I wasn't aware this was likely - I simply let the spring put tension on the belt and then tightened the bolt. The spring as well as the idler are new. I'll have to think about that one for awhile... Keep in mind that it doesn't do it while idling or taking off slowly - only when the engine is accelerated a little - slightly more than feathering it off idle.

I can't help but think it might be something else, but I'm not ignoring your advice...

take off your drive belts and see if it still makes the noise

Check for a crackhole in the accordion hose from MAF/air cleaner to throttle body. Change out the hoses to and the pcv valve. Change the hose from valve cover to accordion hose. Check for a tear in the air filter element. Look for small trapped gremlins.

Get a mechanic's stethoscope for about $20 and go to hunting on the idling motor. You'll be surprised what you can hear, and how closely you can track/locate noises. A poor man's substitute is a wooden dowel from engine metal to bone behind the ear, or a piece of vac hose to the ear, fishing with the other end.

One last idea. What about a belt rubbing under accel forces. I wonder, does it do it when you accel in reverse?

Yeah, I'll try that - just haven't done it yet. That was going to be one of my first steps.

Fasteddy: It does sound like a hole in the accordion to the breather. I can't imagine it being quite this loud, though, but it does sound like it. I've already replaced the hoses from the pcv and valve cover to accordion. Don't think there's anything else to replace...

Keep the ideas coming, guys. I don't think it's a belt or idler...

I tried changing one of the idlers and that wasn't it. It sounds like it might be coming from the back of the engine. Anyone ever heard of a throwout bearing doing this? I replaced the throwout, so I would hope that isn't it, but it sounds like it might be coming from the back. Just not sure...

Anybody got any more ideas? I'd sure like to get to the bottom of this, and I'm not having much luck diagnosing it myself! <img src="/forums/images/graemlins/confused.gif" alt="" />

Partially blocked catalytic converter? I had a car years ago with one,.....made a whooshing/whistling kind of sound under hard acceleration

BC in Austin