Its most likely the AMP. They are prone to failure. It *could* be the head unit, but thats less likely. However, if so I think I have one that works that I can give you, just cover shipping. Something to keep in mind.
How do you test the AMP? Well, no easy way unless another Mark with the same system (there are two versions) is around to do a swap with.
Mine has exact same symptoms. I ignore it, as its only a nuisance at low volume. Only thing i was able to cure was an annoying 'hum' and cutting out - solved that by disconnecting the phone module.
I have a factory amp outta a 97 if you guys need one... just took it out a month ago and it worked just fine... I'd only charge you the cost of shipping... so PM is you're interested... but I agree with Gman... gut and put in a new system... for 650 I got a REALLY nice sounding system that I've gotten a lot of compliments from... I did shop around A LOT!!.. but it was way worth it!! and these cars are pretty easy to set up... I had two friends that never installed a stereo in their life helping me... sure they just stripped and ran wires for me but it took around 8 hours for all new speakers two amps and a sub... all new monster cable wires and the whole nine... do yourself a favor if you're gonna wanna keep this car at least upgrade the deck and amp!
john what year is your mark?