Devon@devons

Hi, have a 2009 ml63amg with an aftermarket stereo head unit that was fitted before I bought the car. Apart from a couple of little thing it works fine EXCEPT every so often it emits a series of loud burping noises thru the speakers, about 15 of them. Then it stops, radio goes quite for a few seconds and then resumes normally. Turning off the head unit or adjusting the volume makes no difference. It has the factory Harmon kardon system. Any ideas please?