87Vettel98

So I have a 1987 Corvette vert with the L98 and 700r4. It has 125,000km on it. The car is a great daily driver but as of recently, I've noticed a noise I can't really pinpoint exactly where it's coming from although it is around the bell housing area. My immediate thought was the transmission. So I did a fluid and filter change to see. I have included a picture of the filter covered in clutch material which from what I hear is pretty normal. Also, this was most likely the original fluid we had it "changed" at Mr. Lube although recently checking the fluid I noticed it was way overfilled by at least a quart and a half. I am assuming they just added fluid to the already filled trans. Anyway, the noise only comes at idle and when you give it throttle it goes away immediately. This happens mostly in reverse but it can be heard idling in all gears even park but once you give it gas it goes away. I thought torque converter or bad flywheel or something of that nature because the transmission shifts completely fine aside from bucking the wheels when downshifting into first which I have also heard is pretty normal with the 700r4 and as far as I know its always done that. I put my stethoscope up to the transmission pan, oil pan/sending unit flywheel dust cover. I also thought briefly that the AIR pump was making the noise but upon further inspection, it was not. Any input on the situation would be of great help. Here is the video of the noise. Thanks,
87vette

81c3

Mine is doing the exact same thing. As soon as I blip the throttle it stops. I think it actually is the air pump... I know its not the transmission or anything inside the bell housing area. Mine only does it occasionally and there are no ill effects other than its embarrassing.... I think its a diverter valve or some part of the air system. I haven't been able to pinpoint it myself yet...It does sound like its coming from under the car but I checked it out last time with the hood open... I can feel the vibration at the air pump outlet and along the passenger side air tube more than anywhere else.

Corvette40

Had the same issue a couple of months ago. Flywheel was cracked around the mounting bolts.

81c3

Was it intermittent, like not every time you drove someplace? Was the noise the same as the video? Did the throttle blip make it stop? Did gear selection make a difference in weather it did it or not? Any other information you can add to help?
Thanks!

ex-x-fire

Unbolt the torque converter & slide it back. Start the engine, is the noise gone or different?

81c3

Problem is, it only does it sometimes... By the time I were to start it up again after doing what you suggest, Im sure it wouldnt be doing it still... I swear when I put my finger on the hose off the air pump, I could feel the vibration there. I was thinking I would pull the air pump and see if it could be cleaned somehow and try that.... Im just not convinced its a transmission/flywheel issue...

s carter

No need to remove the pump it self to rule out all of the drives Just pop off the Drive belt to rule out the front of the motor, do it with motor Cold and run long enough to get answer no more, NO BELT NO COOLING SYSTEM

87Vettel98

I checked my flywheel and no bolts were backing out. I also inspected it for cracks and nothing. The teeth are lightly worn with 4 or 5 rebuilt starters through it. but no cracks and no crazy run out on the wheel. I was kinda wishing it was that and not a trans issue. Hopefully just the AIR.

81c3

Its VERY intermittent... Sometimes it doesnt do it at all when I take it out... sometimes it does, but it goes away quickly. Its never done it when cold either...

Are your symptoms the same 87vettel98?

C409

..... There is an A.I.R. diverter pipe that attaches to the Catalytic converter ... my guess is that it is broken or disconnected .....

87Vettel98

The AIR diverter is disconnected at the cat or lack of one and that is around where the noise is coming from. I think the AIR may be the culprit but I'll check the diverter next time it happens. Thanks everyone for the replies.

81c3

Is yours doing it intermittently? or is all the time?

81c3

So Im pulling this back up because its still happening. Im 99.99% this is not transmission related. I believe its something to do with the diverter valve/check valve that goes down to the cat. I checked the connections and its tight at the cat and up by the air tube.

In an effort to NOT throw parts at it, Ive been looking for other folks who have the same issue. I looked all over Youtube and theres not even one...The video in this thread is an exact match to my issue though. I tried PMing the OP, and never got a response.

I guess I could try replacing the one check valve and see if that works.... but I really dont like this method of repair..... any other ideas? I dont want to remove the system, so I need to fix the issue. Its SOOO embarrassing when it happens... Its always in a idle or while Im parking situation... ie: very low engine rpm or load...

Its never once happened while cold.... always when the engine temp is at around 200 ish... Happens no matter if in park, neutral, reverse or drive... but never while cruising down the road...

confab

I've diagnosed A LOT of weird noises with a cheap Craftsman stethoscope. You know, the ones for cars with the steel probe on them. Think I gave 15 bucks for it, or something close to that.

You can run down REALLY hard to find noises with it if you're careful. Bottom end sounds bouncing around in the oil pan that sound like the trans. Power accessory noises you would never, ever guess by hand spinning them.

Try that, imo. I love mine.

VikingTrad3r

in your video...which noise it it? the glug glug glug glug that you can hear more clearly towards the end after the super high rpm compressor-like sound stops?

C409

..... Catalytic converter ? ... The strata has been known to break up into chunks and clunk around inside the case .....

81c3

Ok, so Im pulling this up in the interest of giving closure to the problem. First I am 10000% certain the sound in the OP's video is the same as mine.

I did a few experiments.... First, I disconnected the rubber elbow from the feed tube running to the diverter valve that goes to the cat. Drove it around like that for a few weeks... never heard the sound.

I figured, why not put this cool little air filter element that I found at AZ, and let it be..... Well, while it looked cool... it didnt sound cool.... the air pump pulsations flowing through the filter made a strange slapping sound almost like an exhaust leak. Off it came.

Next, I tried a hose between the tubes, no diverter valve... No sound.

Now, in the interest of making it look factory, I gutted out the diverter valve to make it a flow through design... Took it out today and no sound.

Im reasonably sure now that it was the rubber diaphragm inside the diverter valve fluttering with the air pump pulsations.... Just worn out more than likely.

So the operation of the controlling device is based on temperature and vacuum. Once the engine reaches about 140 ish it seems to actuate the control valve letting the air flow to the cat. Thats why I never heard it at cold engine temps.

The noise it was making is gone now... and Im glad. I know there was a check valve action inside the diverter valve, but I cant see why. I dont think there could ever be enough back pressure from the cat that it would make it all the way back through the tube into the pump.... That tube is 1/2 pipe and about 6-7' long from the outlet on the cat to the pump.

Ill keep an ear and eye on things for the near future and update if anything happens.