I have had a rattle in the back of the car since about 5K miles. It sounds like a muffled medium rock in a shoe box. The dealer told me I had a bad valve in the rear struts, so they replaced both of them. The sound is still there though. I think it sounds like the muffler bouncing around, but when I get under the car and try to move it around it doesn't.

Any Ideas? Yes I checked the space and the rack!

 

I have a similar rattle, but only when I'm starting the engine, and only when the parking brake is on . Weird, huh? I'm thinking muffler or tailpipe. I figure I'll have the dealer look at it one of these days.

 

I seem to have that same rattle also, but I just try to ignore it. It isn't that noticeable to me unless my car is completely quiet (no music, talking, etc.)

 

Recently I've started to notice a thump in the trunk exactly like you described. "It sounds like a muffled medium rock in a shoe box." This may sound like an obvious and stupid question, but what do you have in your trunk?
This was bugging be today on my lunch break so I fiddled around with my subwoofer box. It made a similar noise if I rocked it back and forth or moved it up and down to simulate a bumpy road. So I pulled the box all the way to the back of the trunk (this was the only place where it didn't make a thumping sound when I moved it around.) And since then I haven't heard the thump...
The bottoms of our trunks are not that substantial and make a dull thump when they are impacted against.
Not sure it'll solve your problem, but at least it's something to check.

 

Nothing in the trunk! I even removed my jack and tire tool to see if it would stop it and it didn't. I am taking the car in for that and the Heat sheild rattle that happens at exactly 2600 rpms every time I accelerate(sounds like metalic bees are attacking). These are loud enough that unless I have the stero cranked I can still here them.

 

i have this same problem , i have tried tightening everything in my trunk, i have tried moving my subs around and everything , i have no diea what it is, the only thing i can think of is when i took the back dash off to paint it maybe i didnt put it back the right way but as far as i know i did , but every time i go over a speed bump i hear the thing unless my music is on, but it gets on my nerves quick , i think im going to have to take it to the dealer but the problem with that is like someone said before, they say one thing and its not that so u end up replacing something u didnt need to ... so i dont know

Danny

 

We just went out in my wife's ES. She hit a bump and there was a rattle. Seemed obvious to me it was either the license plate or frame. You may want to check that out. I took mine(frame) off my P5 because it rattled so bad when the hatch closed.

 

i know its not that in my case cuz i got it extremely tight and i also dont have a frame ... but it does make sense and it does sound similar to what that would sound like

Danny