does the noise go away when the car heats up? did your old H/U do the same thing?

 

JOhnvaroom is completely right it is moisture on the amps capacitors that causes the noise, as it warms up the high pitch squeal should dissipate. If it is a real problem for you get a incar heater to plug in at night to keep the frost off the amps. If you want a cheap fix power down the system before you park the car for the night and don't power it up until the car is warm.

 

A capacitor to grounf acts as a lowpass filter. This is because a capacitors reactance (its impedance with respect to frequencey) is derived by

Xc = 1/(2(pi)fC)

Xc= Impedance of the capacitor
f=frequencey
C=Capacitance

so as you can see it is inversly proportional to frequencey, as frequencey increases the impedance decreases becoming a viable path to ground for high frequencies

 

Yup blame it on the cold battery