I have owned a 2000 C5 coupe std.V8, automatic for the past 8 years. During that time it has had a vibration/rattle at 850-900 rpms that is not motion related. It is always heard as you are coming to a stop and the rpms pass through the 850-900 range; the noise stops below the 850 rpms. The noise is much more prevelant when the A/C is on(which puts additioinal load on the engine). Above 900 rpms it is not heard. Trying to solve this I have had the car to three capable mechanics; one thought the noise was caused by the power steering pump and replaced it, did not help. One replaced the catalytic converter, not it either. One suggested that the left muffler was coming apart on the inside, not it either. I have now taken the car to another shop; they showed me a vibration damper that is bolted on the left side of the differential assembly and it vibrates at the 850-900 rpms;its purpose is to quiet the harmonic vibration set up from the engine. Is there anyone out there that has had this problem and was able to find a solution because the dampener does not do it(it is about the size of a coffee cup or large fist, is an angle iron with two 3inch donut type things bolted to it. Totally frustrated and about ready to sell the car as it takes the enjoyment out of driving it. HELP

 

first where are you located?

sounds like vacuum leak or you have an air leak causing a rough idle since you said it is not motion related....just a guess

 

Are you talking about the torque tube driveshaft couplers? That is the only thing I can think of from your description.
Chris

 

You said it is not motion related but it happens when you come to a stop? Will it do it when stopped and you run up rpms and then drop thru 850-900? That info would be helpful.

PS: How many miles on the car?

 

Can you replicate the noise/ vibration when the transmission is out of gear and OR can you put the car on stands and run the drivetrain up daynamically and reporduce it?

I agree,, Post where you live. Someone will help you figure it out.

Some thoughts::

- Exhaust piping contact frame /body
- Loose brackets
- Aluminum under body shielding vibration.

If you get the car up in the air and crawl under and run the drivetrain up to speed where the issue happens, you will be surpirsed at what you can find..

 

53000 miles. Yes it does it at run up if in gear, worse with A/C on which puts more load on engine. Pretty sure the issue is the dampener on rear of left side of rear end, but don't know what to do to modify.

 

No the vibration is not there when the car is in neutral. I checked the things you mentioned above, thanks for the thoughts. We are in Naples, FL. The issue seems to be with the balancer on the left side of the rear end, but do not know how to modify to increase the absorption of harmonic vibration.

 

We are in Naples, FL. It does not idle rough below the 850 level, low idle is 600. Thanks for the thoughts.

 

i have a similar noise I believe, but mine I kinda confirmed it was the ALT pulley, might want to check it as mine doesn't do it on neutral or park either?

 

Doubtful

If the car is not moving, then nothing in the differential is moving either.

Torque converter is suspect since you say it doesn't happen in neutral.

 

Item is bolted to the outside of left side of rear end.

 

Yes, I know exactly where it is. And I will reiterate, that is for dampening the harmonics generated by the differential. If the car is not moving, then the internals of the differential are also not moving. Thus it is a mystery why you have decided that this is your culprit especially considering it has no moving parts.

 

I've just recently pulled my diff for a gear swap and wondered what the heck that was. Looks like nothing more than a counterweight.

 

I would like to know if you remove the A/C belt and go for a drive if you still have the problem. We need to eliminate all the easy (no cost) things we can first.

Also, we need to know if you have any trouble codes showing. Have not seen that info in the post

 

The mechanic removed the dampener and the vibration got worse; also holding firm pressure on the dampener reduced the vibration( holding hand pressure on the dampener while it was in gear and running on the rack-reduced the vibration also). The intent of the dampener is to deaden the harmonic vibration that is being transferred from the engine down the driveline( this according to a GM retired GM engineer friend). His idea is to modify the dampener or find the source of the harmonic transfer(not to easy). Holding the brake with the A/C on and running the rpms beyond 900rpms eliminates the vibration. With the A/C off the vibration is just barely noticed at the 850-900rpm. Thanks for trying to help me solve this.

 

The noise starts as you decelerate to a stop and decend form perhaps 1200 down to low idle at 600 but is only really heard at 850-900 rpms, and is very loud and annoying at that rpm range.

 

Pull the AC belt off and see if it still does it. You might have a bad clutch or bearing in the compressor.

 

that's how I found out it was my ALT pulley

 

Sounds like you have a torque tube issue.!!

BC

 

I considered the torque tube also, but I was not sure using the A/C could cause such an increase in the vibration in the tube. That was why I wanted him to remove the A/C belt. I would like tearing into the driveline to be the last thing in this root cause analysis once we have exhausted all other possibilities. Hopefully it will be something a lot less serious.