New-to-me '08 GT manual 'vert has a rattle from the rear. Sounds like a cross between marbles in a can and the creak you might hear from a noisy spring. Only happens when warm, like after driving at highway speeds for a couple of minutes.

On the highway, the noise goes away if the road is smooth and you stay at a constant speed. But it comes back immediately if you go over some minor bumps or accelerate or decelerate. Basically, anything other than steady-state will cause the noise.

Also happens around town when accelerating from a stop (only when warmed up).

I went over everything under the car with a fine tooth comb today to see if I could find any loose exhaust hangers, etc. No dice.

Next step is to pull the rear end cover, but in the meantime, anyone have any thoughts/predictions?

 

It's most likely the exhaust hangers rubbing on the mounts. The exhaust will move a lot when it heats up, so looking at it cold, it will look like there's no way it could make contact. Is the exhaust stock?

 

Brought it in to the dealership so they could diagnose. They claim it's a bad muffler (loose baffles). I'm extremely skeptical about this since I've heard rattly mufflers before and this doesn't sound like one. Plus the crunching/rattling noise doesn't happen when the car is stopped - you can rev it all you want and it won't make the noise. It only happens when moving.

moosestang, when you say hangers rubbing on the mounts, do you mean the mounts moving so far that they contact the edge of the hanger? I shoved the exhaust around 6 ways from Sunday while I was under there a few days ago, and did not see anywhere where anything could possibly be making contact.

I'm at a loss at this point. I guess I could try new mufflers just to prove the dealership wrong. Then take it to a speed shop that has some knowledge of domestic cars (not easy to find around here).

Nothing worse than not being able to enjoy a car you just bought.

 

That's a tough one.
I would check the rear brakes to see if anything is loose or sticking (not releasing properly). Dragging brakes can make some odd noises.
While you are there, try wrestling around on the axle to check for anything loose. If the axle moves you will have a bad outer axle bearing.
Also, check your rear ends fluid level, that could be part of the problem.

 

That's exactly what I mean. The steel rod welded to the exhaust pipe is moving enough when hot, under load, to contact the metal part of the hangers attached to your car. I used a grinder and notched the stock hangers(not the steel rod welded to the exhaust pipe, the stock hangers with rubber bushings).

Try wrapping some duct tape around the steel rod where it could touch any metal. Believe me, the exhaust moves when you accelerate, you can't move it with your hands.

 

moosestang, looks like you may have called it! I was looking at the hangers again today and then it hit me, the mufflers move fore and aft as well as up and down/left and right. Tried your duct tape suggestion and danged if it didn't seem to have worked. I'll take her on a longer trip tomorrow, but so far so good. Now I just need to figure out a somewhat more permanent solution before my 1500 mile road trip next week.

Thanks!

 

Hmm, I'm glad I found this thread. My 2010 does the same thing. I took the car to the dealer and they couldn't find anything. The rattle only happens when the car is warm or at normal temps. It also will rattle in the rear when hard accelerating. This has been happening for 2 years now. I did have some Magnapacks installed when the car was 500 miles old. Could my problem be the same?

 

All it takes is 30min of time and some tape to find out

 

So do I wrap the entire hanger with duct tape? I'm mechanically a blonde!

 

My Frankenstein exhaust was hitting the back of the underside of the trunk. Took me forever to diagnose it. I ended up wrapping that part of the pipe with header wrap. Now when it moves if it hits there is no metal on metal clunk.