when its on it thumps good but when i turn my stereo off its starts to hum sometimes pretty loud like a olng steady bass note and sometimes its a little lower
The same should happen with your speaker, you probarly just can't hear it. It's just static mate. Try this- unplug the jack, put the tip to your finger, (whilst your speakers are on, or off) and hear the diffrence.
Just unplug your speakers from your stereo when it's off. That'll make sure nothing comes through.