I fitted an aftermarket HU to my Octy VRS 2+ years ago and wired it up via the PC5-52 and PC2-69-4 leads.

Just before Christmas I noticed that reception on FM was deteriorating and Five Live on AM is unlistenable!

Sat in the car with it out of gear on tickover or with the engine turned off is ok, but as soon as I put it in gear and apply revs you get the high pitched noises coming through.

Has the PC5-52 gone wonky or is there some other form of engine suppression system on the car that could have failed?

Has the PC5-52 gone wonky or is there some other form of engine suppression system on the car that could have failed?

Hit the nail on the head. The PC5-52 could have lost its power feed or is faulty. Also, there could be a split in the aerial wire somewhere touching the chassis and causing interferance.