I just purchased a 98 SS and wasn't sure if this is normal. The car is an automatic and when i really get on it, the car makes a pretty loud noise in the rear of the car during launch. The loud thud almost feels like somethings going to drop from the rear end. This only happens from a launch at a stoplight or very low speeds. It never seems to happen if I'm doing over 35mph and decide to accelerate hard (smooth shifting and no noise). The best way I can describe it, is it feels like the rear end can't handle that much low end torque.
I've never had a car with 300+hp so wasn't sure if this is to be expected.
You think its a tranny or rear suspension problem?

 

Broken/torn transmission mount. Your rearend has a long torque arm that also mounts to the tail of the trans. Under torque, the pinion end of the rearend wants to lift. That makes the arm pull up on the trans, and eventually fatigue and tear the trans mount apart. Then the trans lifts up and bangs on the tunnel.

 

Thanks! Guess I shouldn't drive it much until I fix the problem. I'm suprised the shop didn't notice it while I was getting the state inspection. Do you know if this is difficult or expensive to replace?

 

I think the mount itself is like 30 bucks?
Really easy to replace.
And it's okay to drive it, just don't launch at red lights. lol

 

thanks guys! I was told to buy an aftermarket one, like energy suspension. Looks like I'll be workin on the car next weekend.

 

poly mount will transfer vibrations better. stock mount is like 12 bucks. takes 20 minutes to change.

 

Some guys seem to pick up more vibes than others. I put a poly mount on my old 96, and the extra vibrations felt were minimal.
But yeah, the poly ones are more durable, have an interlock, and won't separate.
Deciding which mount to use comes down to what you want to put up with.

 

This is not to begin a flame war rather a curiosity

why would you think a shop should check a transmission mount during a state inspection?

 

I was always amazed at what people thought we did during a $19.99 State inspection. I had one customer yelling in the shop because he had a State inspection on Friday and the timing chain broke on Monday. How could we miss that? After we explained that was not part of the inspention he switched to, "you did something to cause it so I would have to come back" at that point the owner asked him to leave and never come back, he even offered to tow the car anywhere intown for free as long as we never see him again.

I have even had people complain their cars still idle rough after their state inspections. That was before emisions, heck the only reason the car need to run was for the test drive.

The most common issue is cars passing state inspection then a month or two later they go for a front end alignment. It is very hard to expalin the differents between "Safe" and returning the car to its original factory specs. Inspection allows for a small amount of play in the steering, if you want to hit factory specs during the alignment a little play is a problem.

Just a FYI at GM vibration training class (yes that is a real class and it was 2 days) they did mention some people are more sensitive to vibration but they also said since these are mass produced car the level of vibration can vary a lot from car to car. Cars are check for a Max vibration at the factory but many cars fall well below that standard and some are right at the limit. Motor mounts are designed to handle the worst case. So if your car was worst case and you put a hard mount in it your going to feel it but if your car had low vibration and you put a harder mount in it you may not notice.

 

I obviously don't know a lot about cars, but i know a simple state inspection won't catch everything wrong with my car. This is my first real sports car and it was a pretty loud noise I was experiencing. Just wondered if something like a bad transmission mount would have been noticeable with the car on a lift during the inspection.