Steve
Had a similar problem on an old HIACE camper. Only did it when hot, damned annoying isn't it. I found it to be worn suspension rubbers. I found that pouring engine oil between metal and rubber helped (all I had to hand as I was on a campsite). Try something like spray-grease. If it gets that bad, change the rubbers or turn the stereo up!
Ian - (#743) Steve Reynolds, 4 Aug 01