Audio Systems: RCA Sub woofer, exact model number, powered subwoofer

Question
I have a powered subwoofer that when not in use makes sounds (like a muffled knocking on a door) for up to a minute at a time.   It is connected to my tuner/amp by one wire, which is attached to the left input on the sub (according to their directions).   I do get sound when it is in use, but these "phantom noises" are somewhat disturbing.  The tuner/amp is turned off, but the power is on the sub (its plugged in).   Any ideas?   Its a RCA I bought from Radio Shack (first mistake), but as I live in a condo, I sent my old really good one to another place I own, where it can be cranked up good, and moved this one which was not being used to my main TV.   Larry

Answer
How old is the unit and what is the exact model number and what is the subwoofer getting signal from? Meaning what is the name of the output on your amplifier or receiver? Most powered subwoofer have an auto shut off switch which stop unwanted noises when there is no sound. Some even have a sensor so that it can be adjusted to turn off when it is not receiving any signal.

The very first thing you should do is disconnect any input connection to the subwoofer. If you still hear those low frequency noises then my guess is that there is an internal problem with the subwoofer that you might want to take up with Radio Shack providing it's under warrantee.

If the noise goes away the next thing you want to do is plug it back in to the subwoofer and remove the connection from the amplifier end. If you hear the noises then it's probably a bad cable and needs to be replaced. If it goes away it means that the noise is generated by your amplifier or receiver. Those noises might be there but much lower thru your other speakers but noticable with the subwoofer.

Don't rule out your source  (CD, Tuner,DVD) a test that you can do by switching inputs on your amplifier to see if it goes away. By this time you should understand that it can be anything in your system that can cause that problem. You just need to use these troubleshooting techniques to isolate where it's coming from.

Good hunting!