The radio unit itself is installed in the back where the CD changer
was, the antenna cable is a bit too long, so it is stored in a loop.
Control unit is seperated with 8 meter cable from main unit and is
mounted underneath the ashtray

D-

My guess is that the whistle is alternator whine, as others have suggested. If
so, it is probably not coming in through the antenna, but through the power
connection to the audio amplifier.

I would suggest using a pair of 10 AWG wires directly from the battery
terminals to the main unit (Fused, of course). Either alternator or ignition
noise might be coupled through the control unit's ground, so try isolating it.
Let it obtain audio, power and ground through the 8 meter cable.

If the antenna is ground independent, make sure you didn't ground it by
accident. There could be noise conducted through its shield back to the radio.

73, Fred, K4DII