Why is my subwoofer distorted and making a rumble sound

This sounds like classic "blown" sub, where the voice coil has warped due to high heat. If it feels scratchy or clunky when you gently press down on the center of the cone when the sub isn't playing, then the voice coil has warped and is rubbing against the magnet or top plate. Only way to fix is to replace the subwoofer and make sure your amplifier isn't sending lots of distortion (especially at high volume) to the sub. If everything feels normal and smooth when you gently press down on the cone, it's possible the distortion and rumbling sound is coming from the amplifier. The easiest way to test that would be to hook up a different functioning sub to the amp and see how it sounds. If the other sub also sounds distorted then you know the amp is at fault. Remember that most woofer problems are caused by distortion coming from the amp and overheating the sub's voice coil. If you fix your sub, also make sure the amp isn't distorting so you don't damage another woofer.