RPM drop and "popping" sound on mag check, C-172

Hi there,

Googling past threads yielded nothing definitive so here goes:

On runup today, noted the following:
1. RPM to 1700, switched to right mag, dropped ~150 rpm

2. Increased RPM to 2000, leaned back until I got a 100rpm drop and
held for one minute to clear it. We've had to do this pretty regularly
lately*

3. During the lean-out I started hearing a regular popping or clicking
noise, clearly electrical in nature, about 1-2x/second on the intercom

4. After leaning out, throttle back to 1700, check right mag. RPM drop
maybe 50-75, popping sound disappears

5. Switch to left mag, popping sound returns, rpm drops 200

6. Repeated leaning out at RPM, same result, i.e., left mag still
dropping too much for me.

I'm going to call the shop Monday to have the plugs checked, is there
anything else this sequence of events suggests to anyone? Airplane in
question is a 1979 C172N, engine tach time about 1800. We do have one
cylinder that tested 63/80 at annual back in March.

* We do lean for taxi, but probably not enough. It has required a
burn-off more often than not the last few months, but one or two
cycles has always cleared it up successfully.

TIA,
-cwk.