Awhile back I noticed that when I shut off my receiver, the subwoofer still made noise (almost a muffled slight rumble.) I thought it might be the cable, so I recently purchased a new Monster cable and it still does it. What might be causing this? Whenever I watch anything with hardly any bass, it doesn't make any noise at all...it sounds like it is off....

Sounds like an amplified ground loop to me. Mine made a low rumble until I got the empeg. At that point I decided to "do it right" and grounded every piece of electonics in my car to the same point, and that directly to the battery. Everything went completely silent.

Oh, and check that you ignition sense wire is wired properly to your amp. Your subs SHOULD be off when the amp is off.

I think he forgot to point out this is a subwoofer in a hometheater setup since he doesn't have one in his car

Ah.... well.... That changes everything. Often times this is caused by a poorly shielded RCA cable running next to a power line, inducting a 60 hz buzz. I realize you bought a new cable, so this probably isn't it, buy hey, just throwing out possibilities.

Is it an active or passive sub woofer? I had a sensor problem some time back where the SW didn't cut off when I powered down, which created just the sort of noise that you describe.

N.B. Sorry, obviously an active one re-reading your original post, but the comment stands.

so which one did you have an active or passive subwoofer?

so do you think it is the subwoofer or the receiver?

I stand by the opinion that your cable run run's net to a powerline, ot there is a faulty shield inside your amp. Definietly sounds like an inducted hum to me.