CX-7 AM radio issues

I know most people these days don't give a rat's a$$ about AM radio, but I like to sometimes listen to it. Problem: In my CX-7 unless a station's transmitter is less than 10 miles away, I always get reception interference from the engine. It's a steady "chik-chik-chik" that runs at about the same audible tempo as an old fashioned analog stopwatch. It's done this ever since I've owned the car -- about 5 yrs ago.

Only does this when engine's running. Is there anything I can do to stop this?
 
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2003 protege 5
I know on engines with spark plug wires if you use cheap wires it can cause radio interference. But the 2.3 engine in the cx7 has coils on plugs with no wires.
Is it a stock stereo? If not could be several things like the unit itself, a grounding wire, or damaged antenna/wire
 
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