2001 cavalier install problems

i installed a deck in my 2001 cavalier and used the "metra 70-2003" wiring harnsess.  everything went fine and the deck turned on and the speakers work fine but, if the volume is turned all the way down or during the moment of silence when you change songs or for a split second when you eject a cd.  there will be a low pitched sound that will come from the rear speakers, sounds like bass coming on and slowly getting louder, like when you hold a microphone close the a speaker and it makes that sound.  what could this be?