I consider myself a pretty good diagnostician, but this has me stumped.
I have a 93 six speed Vert.
When I accelerate in any gear and put a heavy load on the engine, i hear a slapping noise that sounds like it's coming from under the car. Backing off the gas a little makes the noise stop, but applying the gas brings the noise back. The noise gets faster as the car picks up speed. It happens in any gear.
It sounds like a rolled up magazine is being smacked against the car repeatedly.
The wheel bearings are new, but I haven't checked the U-Joints. I've got to believe that it's some part of the drive train making this noise.
I'm getting new tires put on soon, so I plan on checking everything I can while it's up in the air.
Any ideas??
Thanks in advance!

 

Check the driveshat U-joints,
And check both half shaft u-joints.
6 total to check.
Unusual problem you have.
But once you get your car up in the air I think you will find out what the culprit is.
Maybe a plastic bag or string is wrapped around the driveshaft or halfshafts.
I have run over those plastic bags in the wind on a very windy day in Illinois.
They whip around in the road worse than leaves do.
Somehow hard as you try to miss them, they blow underneath your car anyway.

Not missing a cat by the way? Some animals love to make camp underneath cars. Including Vettes.

I have seen that. They are always dead.
@#$%%

Brian

 

It isn't detonation you're hearing is it?

 

I'm not missing a cat, but I haven't seen my neighbor's cat for a while... Actually i pulled a big mouse nest out of the space between the radiator and the AC condenser this past summer. When it's in the air, I'll try to check out the u-joints.

Detonation?? I can't rule it out, but it sounds external to the engine. If it was detonation, what's the cure? I'm using high test gas. Would I have to remove the heads?

Thanks for the ideas.

 

This systematic diagnostic approach was taught to me many years back.
How they used to teach all young aspiring to be GM Master Mechanics.
They quit teaching those young mechanics(all of those guys in their 20's today) right when Hinsdale GMI closed in Chicago.

They get taught the "AUTOZONE" way now...parts changer.............

1) Verify the problem or customer's complaint.
2) Isolate the problem.
3) Repair the Problem.
4) Recheck Your Work. Is it the repair done correctly and to the best of your ability?
5 )Road test your work and be sure the customer's complaint is gone.

Same ideology holds true today.
Our C4 Vettes included.

Brian

 

One way to test for detonation would be to add a few gallons of unleaded race gas (100 octane) when the tank is low. Hammer the throttle. If the noise is no longer there, you've got a detonation problem (too much timing may be one cause). If the noise is still there, look elsewhere. Sometimes detonation can be pretty loud and sound like it's outside of the motor. If it is, figure out the cause or you will soon need a new motor. Willie

 

Do you feel it as well as hear it? Could it be the exhaust hitting?

 

Sounds like your saying the speed increases with car speed not engine speed, if that is true, you just ruled out 'pinging' since it would change speed as the gears change. Anything on the drive line will continue to increase in pitch as the car speeds up regardless of what gear it's in.

IMO I'd start where Brian suggested, check your u-joints, transmission output bearing and diff bearings for slop.. Remember the speed of these, 1/2 shafts, 1 rev per tire rev, driveshaft around 3X faster than the tire.

 

Check engine/trans mounts

 

Thanks Willie and other posters. I'm going to take a two pronged approach here- one is the physical slapping I'll look for, but the other is the engine. I never thought that it could be detonation, but this morning when I drove the car, I didn't hear any noise (and couldn't cause it by acceleration). the engine was cold. later on after the engine had warmed up I could cause the noise to occur by loading the engine. Sooooo, high octane gas, here I come. It's an inexpensive test (well, getting more expensive every day with $4.00/gallon high test). I'm already using premium, but I'll try to find some Sunoco Ultra or similar.

I just ordered some tires from Tire Rack so the car will be up in the air soon and I can check all the moving/rotating parts.
I've installed ujoint kits on a BMW 2002 (long ago). Can I assume that the Corvette U-Joints are not to bad to install?? (Even if they aren't the cause of my problems, this car suffered from some neglect over its life. I've owned it for a year now, and these would be a logical project).

I'll keep you posted on the results of race gas and other observations.
and Brian- thanks for the advice on "First Diagnose, then Medicate." Makes sense to me.

Paul

 

Leaking exhaust or header?? That can get pretty loud/harsh

 

I recently had a ujoint problem and was advised to spray the ujoints with some PB blaster or some other lubricant and take it out for a drive to see if you hear the sound. Do it one ujoint at a time, that way you can isolate which one is bad, if any. I did this and it worked like a charm, so I replaced the bad ujoints and I'm on the road again.

 

I had a slapping noisefrom the wheel bearings for a time, seems when the grease got hot, the bearing ***** slapped together in the bearings. I wasn't able to disect the bearing, but changing the right front fixed it.
Since you are hearing it on increased speed, I suggest a right rear bearing. Remember, each hub has an inner and an outer bearing in each one.

 

While you have it on the lift check the entire length of the exhaust for shiny or scratched up spots where its contacting somewhere.

 

You are a Very Smart Aviation Mechanic and C4 Vette owner.

I have heard wheel bearings failing a few times that made a funny light slapping noise.
The internal cage holding the ball bearings in a clocked and uniform position fails or breaks apart.
The grease dries up and dirt do their damage over time.
The ball bearings then roll around in a non uniform pattern.
They bang into each other when rotating down the road.

I have only seen that a few times in front wheel drive cars.
Mainly Japanese built vehicles that use a separate pressed in double row ball bearing in the front spindle assembly.

Never seen a similar failure in a C4.
But it is possible.

I find many hard to track down noises from the drivetrain with a Mechanic's Style Stethoscope.
Like old doctors use but with a prodding metallic tip.

With the car off the ground, preferably a 2- post lift or 4- post for extra safety and ability to walk underneath......

A helper is inside the vehicle.
He starts it and puts it into gear.
I have him accelerate from 5 mph to 60 mph gradually.
I walk around underneath with the mechanics style stethoscope checking for noises.
Touch the engine oil pan lightly, engine block, different parts of the exhaust system, transmission pan, torque converter cover, transmission tailshaft, rear differential housing, and driving wheel spindles both.

Never prod around listening with the stethoscope on any rotating exposed parts of course. Like driveshafts. Or half shafts on a C4.
If you do accidentally, the rotating part will whip the stethoscope out of your ears, and the Very loud noise will leave your ears ringing.

Happened to me once listening to a rear differential under power on a lift. Checking for a rear differential pinion noise(s).
My Boss came over and slapped me on the back to get my attention.
Wanted to borrow a tool from me.
He surprised me and I jumped.
Stethoscope hit the driveshaft.

I was deaf for about 5 minutes.

My boss is a piece of work too.

Brian

 

Could be your timing chain