I installed a Pioneer flat screen GPS unit in my Chevy HHR and was getting the same "whizzing" noise and sometimes it would be constant, sometimes vary with engine speed. Sometimes it would not be there for weeks? I ended up doing what the second poster suggested and it worked. Sometimes it does come back and the shop I bought the unit from said to pull the head unit out and open and reclose each RCA plug, twisting it as I reassemble. That fixes it for 6 months or so.
The stereo shop I bought my unit from carried the filter.
