Can you give me the year, make, model, engine size, and a general guess as to where the sound is emanating from? I don't need you to fully diagnose it yourself and give me the component source, just a general region of the car. Also, is it worse while driving, or climbing a hill? Edit: stick, or automatic. And is the car in or from a region prone to rusting?
1996, Honda Accord, 2.2 liter I4, seems like it's coming right from under the shifter (automatic) and a foot or so forward. The sound is the same no matter what stress the engine is having. EDIT: I live in the midwest, minor rust, only on really old cars that sit outside constantly, mine does have rust on the bottom part of the rear doors and wheel wells. --- Post updated --- I live in Utah
Crawl under it sometime and see if the metal shielding around the catalytic converter is rusted to junk/ loose, or if any part of your exhaust track is touching something solid. It isn't likely to be anything super nasty because it is tied to the engine, the sound emanates from somewhere other than the engine, and load has no effect. Bottom end engine issues are either constantly present, present only with a fully warmed engine, or tied to engine load. Transmissions don't make noises like that without being completely grenaded inside, and anything that rotates with the engine running and the vehicle stationary would make a constant noise or react to load. Your sound is related to resonance, and is unlikely to be any mechanical failure based on the information you've given me. If it is the cat, you'd be best to either rip the sheilding off (without puncturing anything!!!) or ingnore it (that sheilding has very little function and will not harm anything if absent or loose). Hope it helps.
Thanks for all that info! Iooked under without the help of another, I discovered a gasket leak on the exhaust, which explains another sound I've been hearing, I didn't see any shielding under the cat, but I'll get someone to help me and find it.
I have the same problem with my BMW, one of the rubbers that hold the exhaust pipe in place is broken, and coupled with my absolutely horrible engine mounts, I get a nice rattling sound when idling. Be sure not to look only at the cat/muffler/whatever is at the end of the pipe, be sure to check it in its entire lenght.