I have 10" Crossfire subs and just recently one of them has started making a sound at very low volume that resembles the kind of sound you get if you run a sub-bass frequency through a stock vehicle speaker. It's almost a frapping noise. If I go ahead and turn the volume up, that noise disappears and the sub still pounds but the SQ is not very good. Is this noise an indicator of a voice coil being\going bad? If not, what could be causing this? I've had these subs for about 2 months now and the noise just started roughly a week ago and seems to be getting worse every time I turn the radio on.