Ugh, this problem happens to me too! I installed an f90BT in my 1997 Lexus ES300 and I can hear any and all motosr that turn on in the vehicle's electrical system. Alternator, fan, CD player in HU, HU fan, etc. etc. It's SOOOOO annoying.
I was thinking about getting a ground loop isolator or choke to fix the problem, but I also thought it may have been due to problems with my factory installed amplifier to factory speakers. Additionally I was thinking about trying out the RCA outs to a 4channel amp and some 6.5"s to see if the noise is still there. The trouble is I don't have an amp and