If it's happening when you press a button, it *could* be the contacts on the back of the faceplate (I'm assuming you've got a face-off headunit here). Try just pressing the front panel, see if it does it then. If yes, clean the contacts up.
Could also be the connections at the back being moved slightly when you press something, so check they're all insulated well...
Other than that, it could be electrical noise generated inside the headunit and there's not a lot you could do about it.
What headunit / amp is it?