| well, would you believe it the whistle returned!!!
so i noticed that when i removed the rca plugs from the amp on 2 channels the noise increased. if i hooked up only channels 1 and 2 it was better. as soon as 3 went in it got loud and when 4 went in the noise remained but reduced in volume. took the amp out removed the circuit board and had a look at the solder joints under a magnifying glass. lo and behold very obvious dry joints with evidence of previous soldering... yellow areas too.
so, i resoldered and hey presto , perfect!
now i just have to get the bluetooth to work.
im going to put the old HU back in and connect it to one speaker (and old one ive mounted under the steering column) if this works im thinking of somehow having both HUs as i dont want to give up the alpine.
thanks for the advice da-bull!
simon | 
