I pulled out of the grocery store today and my car sounded like it was dragging something - almost like tin cans from a “just married” thing. I checked underneath and couldn’t see anything, but short of Mickey Mouse popping out of there, I wouldn’t know if something was unusual. I have a friend who knows about cars and drove it up to his place, the sound got louder - it is particularly bad when I am decelerating or just going slow. At those times, it sounds like rocks rolling around in a metal barrel. At higher speed, it sounds like hockey cards in bicycle spokes. My friend is working nights and will not be home until 4 am. I am supposed to drive my car pool guy to the airport tomorrow morning at 6 am, and then drive myself 150 km to work everyday for the next 2 weeks before he returns. Am I screwed?? Please tell me this is either (a) not serious and can wait or (b) inexpensive an quick to fix. HELP!!!
There’s no way to answer (a) or (b) without first knowing what the problem is.
Do you have a section of tailpipe or muffler dragging on the ground or missing? I’ll assume not as that would be pretty obvious. Can you tell whereabouts the sound is coming from…under the hood, under the seats, from a particular wheel, etc?
Other wild guesses are a bad wheel bearing, a cooling fan loosening up and rattling about, or a REALLY trashed CV joint.
It seems to be coming from the rear of the car. Had a bad wheel bearing last year repaired - this sounds nothing like it. Nothing is hanging down on the ground or even appears to be out of place. I’m pretty sure a CV joint was part of it’s recent service (just spent $1700 on it for 200K service plus a bunch of repairs including brakes and possibly a CV joint).
I should say I hear it better with the window open than closed. I don’t know if that helps but I’m pretty sure it’s not under the seats.
Okay, another thought is an exhaust heat shield that has come loose and is rattling against the body/exhaust. Sorry I can’t make more than educated guesses, you really should have a mechanic listen and diagnose. Tough to do that over the internet. 
Is that something that could happen suddenly without any noticible event? I didn’t hit anything or run over anything and had been driving approximately 25 km stop and go prior to the grocery store. I believe I had my window open the whole time but I can’t be 100% sure. I didn’t notice anything until I pulled out of the grocery store. I’ve got it a friend’s house now - the sound got worse as I took it there. He’ll be home at 4 am. I urgently need the car at 6:30 am. Is it hopeless?
You might tell us the year, make model, mileage…
It’s a 2003 Toyota Matrix with 205000 on it (yes I commute a long way and I am supposed to work tomorrow AND drive someone else in).
When the car is cool, grab ahold of the tail pipe and give it a good shake. Maybe your muffler’s come loose.
do you routinely run over gravel roads?
frequently the debris shield/cover over the catalytic converter gets loose, and small stones can get in there and they rattle around just like you describe. if thats the case, its not really a problem. the solution is to poke the rocks with a screwdriver and pry them out of the shield.
that said, there are some other possibilities, but from what you describe this sounds about right.
If the wheels have plastic wheel covers, a lug nut may have broken off and be rolling around inside, between the wheel and the cover.
oh yeah, rocks can get between the hub caps too. usually they find their way out, but hey there’s always a first time to capture rocks there too!
does this make the sound when stopped, while revving the engine? or is it just when wheels are moving?