hi all. I’ve recently changed the front speakers in my 2004 b6 a4, which has concert with half amplified rear speakers and subwoofer. I also replaced the headunit with an alpine cde178. Since being installed there is now a high pitched hum from the rears, the tweeters I think. The noise does not vary with engine speed or volume, it is constant when the headunit is on, whatever the source. Has anyone come across this? Is there a problem with upgrading front speakers if they are rated higher than the old ones? Any ideas would be appreciated. Thanks