VolVette

As the topic suggests, when I accelerate from either a static position of a very slow roll, I hear a metal on metal grinding noise. I have three suspects:

1) Exhaust: I recently had 2.5" true duel exhaust installed and it hangs VERY low. (due to poor installation that I have brought back 3 times! :mad ) When I accelerate the front lifts and the back drops and I think my exhaust is scraping. It does scrape fairly often when going up steep slopes or speed bumps. Has anyone else experienced this?

2) Transmission: Is the original automatic mated to a ZZZ crate engine. Never had any problems. Fluid level is fine, does not leak.

3) Differential: Also original, never leaked, no problems, is 3.08

Can anyone offer any advice? Thank you.

Cheers!

flood

I vote for #3

The only reason is because I had the same problem. The yokes were rubbing on the side of the Diff case.

page62

What about rear wheel bearings???

Dwayne 76

U-Joints????

tictocdok

on mine, after I drive it a few miles the rear rotors hang up... stop at a stop sign and when you take off you can hear the rear pads chattering. I think my hard lines are rusted on the inside and pressurized fluid gets trapped on the rear calipers. I changed all 4 calipers, the master cylinder and all 4 rubber hoses next will try new lines.

mine doesn't do it if I shift into neutral,coast to a stop (no touch the brake), and then take off!!


[Modified by tictocdok, 6:16 PM 7/8/2002]

JMV

Not sure what it is, but you can easily check for #1 by just putting a piece of tape on the lowermost part of the exaust pipe (I assume there are already scrape marks there) and checking to see if that is scraped after take-off. Good luck!

73454

Mine started making what I thought was metal to metal noise once, come to find out a motor mount broke and the fan was chewing the shroud up, I would not have guessed that it could be that simple. You may want to check yours just in case.

STW!

Linwood

VolVette

Thanks for all the good responses! Any other experiences?

I will have to give some of these suggestions a try or two. I hope and pray it's not the tranny or differential.

:cheers:

VolVette

I gave it another road test.

I put some tape on the places where the exhaust would scrape and it came back clean.

The brakes do not seem to be the problem either.

The differential may be the problem. But can you help me figure this, when I accelerate hard (as in the kind that burns rubber) I don't hear it, but when I accelerate moderately, like a stoplight, it makes the noise?

Any help? Thanks guys!

JMV

Did you check the fluid levels in rear axle and transmission? May not hurt to drain and change. :yesnod:

Tron79

I'm going through the same grief right now. I've had a 79 for about 3 months and this is the biggest issue I'm having. I ordered new U-joints today, and I'll see if those help. I have noticed a slight leak around the left yoke seal in the differential, so matters may be worse. The grinding I experience only happens slowing down to a stop or pulling slowly away from a stop, like you mentioned. It also doesn't really happen till I've been on the road for a few minutes. :confused:

Corellian Corvette

Had a similar problem in my '73 and, interestingly, in my '99.

Since it's hard to describe sound, here's a cheap and easy fix (can't hurt).

Go to your chevy dealership and get a couple ounces of their limited slip additive.

There is a wierd chatter/grind that, for some reason, stops with this fluid. Again, had the same problem with both cars.

If the fluid fixes the problem, you may want to spring for the $30 to have your rear dif fluid sucked out and replaced (be sure to put the limited slip additive back in!)

V8yunkie

Check the mount of your 2,5" exhausts where the pipes enter the transmission crossmember - in order to center the tubes correctly in the two holes of the crossmember, you need to modify the old 2" hanger or rather buy a 2,5" hanger (used in 70-74 454 BB's). Otherwise (and even more if your lefthand motor-mount is bad) you will here a noise during acceleration or on heavy road-bumps.

Had the same problem with my 2,5" chambered exhausts.

Good luck!

VolVette

I have always suspected that my differential fluid may be the problem because last year I brought a shop some Mobil-1 Synthetic Diff. Fluid and the Additive and had them change my fluid. I always suspected that they may not have done a great job, mostly because I didn't do it myself. I may have to give this idea, as well as the others, a try.

Any other ideas?

page62

There seems to be an obsession with synthetic lubricants on this board... :nono:

Replace the fluid with gold ol' Dinosaur gear lube, plus a bottle of limited slip lubricant. Then report back to us.

JMV

I seem to remember another thread similar to this where the syn oil didn't combine well with the posi additive. So he sucked it all out, replaced with dino oil, problem solved :cheers: Hope it works for you, Stephen. :D

Tron79

Greetings. We had a hot (and bright) day here today, so I was able to get the new u-joints and get up inder the car where I could see real well. What I found was that the bolts holding the u-joints to the flange (connected to the differential) had been hitting some kind of metal "stop" on the side of the differential. Apparently, certain levels of torque must cause the rubbing. Anyways, I also found that it looks like the differential seal that the yoke sticks through is leaking/gone. So, looks like I'll be taking the differential out to replace the seal, at a minimum.

Anyone have any suggestions? Rebuild? Replace? Just put a new seal in?

Thanks :(

JeffReed

I'll throw something in totally different. When I got my 68 on the road, I had this really nasty grinding sound when I accelerated from a dead stop. We checked all the things posted by the guys here and it didn't help. It turned out the driveshaft hitting the tunnel which wore a hole in the fiberglass, right behind the seats. A Corvette shop figured it out. They said there was a strap or something missing that held the differential in place, to keep it from rotating. I can't remember exactly was it was and its hard to see from underneath the car. Maybe someone else can put some technical term on my rambling. Good Luck!

V8yunkie

Tron79:

Replacing the seals only might not help cause usually, the seals produce kind of groove into the yokes surface. So the new seal will again run in that groove and therefore never be able to seal tight.

Either you replace the yokes ($$$) or shim the seals in the housing in order to position them in a way that they contact the yoke about 2mm beside the groove.

Tron79

Good Points. Thanks. At this point, I'm thinking about a new Diff from Zips along with new yokes and a rear-end rebuild kit. But, I'll wait till the weather prohibits driving.