It dosen't do it all the time, it only happens when I am at a complete stop with the brake pushed in. It idles at about 700 and the back of the car makes a rattling noise and there is a slight vibration throught the whole car, which seems to come from the rear. If I let off the brake or put it in neutral then the idle goes to about 850-900 and it dosen't rattle or shake anymore. Does anyone know what the problem could be? It is a 1992 3.1L automatic firebird with a flowmaster muffler. Thanks for any help in advance.

Jeff

 

harmonic balancer? motor/trranny mount?

hell, just let TomP guess it

 

Check the exhaust. It may be hitting someplace on the body/chassis. Go under the back and grab the tail pipes or muffler and shake it. It is hitting anywhere? Can you see marks where it may be hitting?

A slim possibility is that the tranny mount is broken (common). Typically this shows up as a thunk during aggressive take-off.

RBob.

 

checked the exhaust and thats not it and i actually just got the tranny mounts replaced. Why would it only happen if it is in drive and the brake is on? If it is doing it and i shift it to neutral with the brake still depressed it stops immidiatly could it be the rear brakes? thanks

 

Motor mounts. Check the passenger side mount...it's probably tore up. The passenger side mount take the most abuse because of the way the motor spins. It will cause bad vibrations at idle in gear but it goes away off idle.

 

How do I tell if the mount is bad? Will it be obvious just by looking at it?

 

Yeah, look at the rubber. If it's cracked, sagging, squished, then it's bad. Sometimes they'll be so bad that the engine mount bracket will sit on the frame. That's when it vibrates real bad.