Stereo Troubles

[CMYAU3]

some times when i start my car the radio turns on but there is no sound,
and after about 2 minuts of driving it will sound will come out.
the bit i dont get is some times when this happens
( radio turns on but there is no sound )
i turn the fan on and then the sound works,
or if i wiiiiind the window down the sound also comes back on.
( sorry cant speel )
has it got somthing to do with the air pressure in the car when i gat in .
and is it just me.
ive got all standard ford stereo ..........forte
all that has been added is a 10" response sub n amp in the boot with a line level converter. and that was all intalled proffesionally. over year ago