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Noise when accelerating

Glide

Well-known member
Joined
Jan 23, 2007
Messages
118
Location
Charles Town, WV
Corvette
1971 Shark, 1985 C4, 2011 C6, 99 FRC. 66 Coupe
Originally I thought I had a bad u joint but they all checked out. My noise, a thumping under acceleration occurs only when there is a load on the drive train. If I either let off the throttle or downshift it goes away. A fellow Corvete owner said it's probably the exhaust rubbing one of the half shafts or drive shaft. I haven't examined the pipes since learning this. Just wondering if anyone else has expierenced this and knows how to fix it? It's the stock exhaust I think. That's what this forums for, right?

Thanks!
 

vetteboy86

Well-known member
Joined
May 26, 2003
Messages
2,760
Location
IN
Corvette
1986 Black "Indy 500 Pace car replica"
I've had a similiar problem with mine. When I accelerate from a dead stop, I have what I call a clunk. I had it a corvette shop only after I failed to diagnose the problem correctly. I thought it may have been suspension, but they said it looked fine. My next idea was maybe the tranny mount was bad, and it was the tranny rubbing. Now I think it might be in the diff perhaps, but I am not really sure. Some noises can be a bear to track down. Maybe some of my ideas will help.
 

3dale1

Well-known member
Joined
Mar 12, 2005
Messages
158
Location
PA
Corvette
65 convertible L-76
My vote goes for a motor mount, but I haven't heard of them being a bad problem on these cars...so how about the c beam?
 

Glide

Well-known member
Joined
Jan 23, 2007
Messages
118
Location
Charles Town, WV
Corvette
1971 Shark, 1985 C4, 2011 C6, 99 FRC. 66 Coupe
Good call 3Dale1. What's a c beam? I think, due to a small amount of play where the drive shaft leaves the transmission, it's rubbing that guard (C beam?) that runs along the drive shaft. I was going to check it my removing it and adding a washer under each mounting bolt (4) thus giving me an extra mm of clearance, Just to diagnose it of course. You check the motor mounts by putting it in drive and giving it gas with the brake on and see if the motor moves, right?
 

Stallion

Well-known member
Joined
Nov 20, 2002
Messages
2,305
Location
Jersey
Corvette
1996 CE LT4
First off, the motor will slightly shift due to the torque even if the motor mounts are okay. Second off, only one motor mount will go bad (as far as wear pull-off goes) because the twisting motion of the motor is down on one mount and up on the other. These motor mounts don't usually go bad, but if you think that might be it then inspect them. They're usually pretty durable though.

I had this problem with a '78 Corvette and I couldn't find it. I mean, it was driving me nuts. Motor mount, nope. Tranny mount, nope. Exhaust, nope. It ended up being that the fan shroud slipped down an unnoticeable amount and when under high torque (acceleration, not just high-rev, because the engine naturally twists and "jumps" a little bit) the fan was pegging at the shroud.

Goes to show you that it might be the thing that you wouldn't even guess. A lot goes on under acceleration. Just another thing to look at. And it's near impossible to isolate where a sound is coming from within the cabin.
 

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