I am having trouble with a recently installed self-powered sub.
When I turn off the ignition the sub emits a loud rattling hum for a second or 2 that slowly fades away.
I've never had a stand alone sub before, so I'm not sure if this is normal or not. By the way this is the Bose system.
Appreciate any help I can get.

I would take it back to the ppl that installed the sub... :confused:

Get your power directly from the battery, also your remote turn on might be the problem, look for another one.

I had the same problem with mine. Here's the low down amd the fix:

Initial install: Power from the battery; high power input (speaker level input at the sub) from the rear speaker wires; ground on a screw near the Bose amp in the trunk; and << remote turn on>> via an electronic switch also connected to the speakers which is supposed to turn on the sub amp on detecting low voltage, anything above 5volts. They charged me $35 for the switch. On anything above 1/3 sub level, there was the vibration you describe and the installer said it was because of some Bose proprietary problem (BS)

I came home, tested the speaker wires and found Bose has 4-5 volts even when the system is off which keeps the new sub amp powered up all the time, so I threw away the switch and connected an 18guage wire from the fusebox, the fuse marked AC( which turns on only when IGN is on), I think 5 amp, (after the fuse of course) and ran it to the remote turn on connecter on the sub amp. Now when switching off the car, the ignition turns off before the ACC and so the sub amp dies before its signal input and -- viola-- no more vibration even at max, garage shaking levels.

BTW I showed the installer (who claimed he could "wire up the F*****G planet") and he wanted to hire me but I told him he couldn't afford me. He took the switch back though.

good luck