Help? low humming noise from subwoofer when stereo is off, ignition on

I have a 12" polk dx subwoofer hooked up to a MTX RT2100X amp.  Recently I began to hear a low humming/buzzing noise from the subwoofer.  This happens when the stereo is off and the key in the ignition is in the on position (but the car is not idling/running), so power is running to all the electronics including the amp and sub. As soon as I turn on the stereo the noise is gone. I also notice the humming noise while the car is actually running and the stereo is off - once again it goes away when the stereo is turned on. I'd appreciate any helfpul suggestions.  Thanks.  

It sounds as though you have a ground loop issue.  I'd be willing to bet that the buzzing sound doesn't go away when you turn on the stereo, it just gets hushed by the active input signal and music.  I'd start by checking all the power and ground connections on your amps first both at the amplifiers and at the connection points.  Then disconnect the RCA wires from the amplifier.  If the buzz sound goes away then you should try grounding the stereo to a better grounding point, the factory grounds are usually not very good at all.  Then try it again, if it is still there try replacing the RCA cable.  There's alot of other things to check out as well, but these are the most common places to start.  The buzzing is usually referred to as alternator whine if it changes pitch with the RPM of the engine.  I would also recommend doing a "BIG 3" upgrade, it's cheap, it's easy, and makes a tremendous difference to the charging system of your vehicle.  It allows you to squeeze every possible amp of current your alternator can supply.  It should be a must have for anyone who has even a moderately powerful system.  Well, hope some of this info helps, PM for more suggestions if you need them.  

Thank you for all the information.  I'll be sure to check all the connections for the culprit.  I don't know if it makes a difference, but I really do believe the humming noise goes silent when the stereo is turned on.  Even when I have the volume turned all the way down or muted on the stereo, the noise goes away when the stereo is turned on and returns once it is turned off. I'll check the connections anyways, but let me know if that additional info changes your assessment of the problem any. Thanks.  

I think I'd still check all the connections.  Most likely the noise is going away because the signal wires are becoming active.  I might actually think the RCA wires are bad and check them accordingly first.  It sure wouldn't hurt to go through and check all the connections anyway, I usually check EVERYTHING when I'm troubleshooting a system even if I've already found the problem and fixed it.  Sometimes I find things before they even cause a problem that way and I'm sure my customer has no upcoming problems that going to bring them right back.  

shane from Cambridge,mn writes,i own a 94' Cadillac seville sls, with the factory stereo,and i have a BOSE factory amp in my trunk-i wired up a outline converter to my 6x9 wires in the trunk to by-pass the amp
(other wise i'd have to re-wire all my factory speakers),but now im getting feed back when i turn my stereo off-i have the remote wire comming off the stereo-i had it off my power antenna first then i switched it to the stereo.I have 2 15 inch kicker CVX's,powered by 400x2  and 2 12 inch kicker CVR's,powered by 150x2-after i turn my car off-it wont start again for an hour or two-I get full power-but it wont turn over-then an hour later-it will fire right back up-i just replaced the starter-THATS NOT IT!!-i dont know what to do?---why do i get feedback humm and why wont my car start again right away-no clicks or anything...shane