Right after me singing the cars praises fro 50k miles of service it's promptly paying me back by gaining a really annoying fault!

When I start the car in the morning all is fine, however as soon as the revs exceed about 18000rpm I get a crackle through the speakers, teh crackle is continuous and does not raise and fall with the revs it's there or it's not. After a few mile sof driving it does settle down but the crackle appears again briefly as the revs rise and fall past the 1800rpm mark. As a first port of call I checked the ground for the amplifiers but cleaning this up has achieved nothing.

It's now bugging the Hell out of me as other than this the old tank is relatively faultless

Radio is an aftermarket Alpine 9812RB