If the reciever is off, then it must be coming from the subwoofer. Give it a listen, and it if is, it can be a number of things.
1.) Bad/loose connection
2.) Power left on and sub picking up signals.
3.) something physically in the subwoofer (most likely not, but one guy had a rat in his
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)
Thats what I can think of off the top of my head.
Do you turn your subwoofer off, or does it have auto on/off?
If you use auto, check to see if it will turn off after your receiver is shut down. Give it a couple mins and it should turn off. If not, theres your problem, and I suggest getting it looked at, hopefully there is still a warrenty.
Sheepstar
EDIT: It can also be the cord, I had a cord that would make contact after everything was off, or not being used, and it would make the sub blare you 60 tones and spirts.
EDIT 2: Do your mains have powered subwoofers in them? Or are they powered?