Matt,

The wire from the magneto to the key switch is called a "P-lead". In
the NORDO days, this used to be a single, unshielded wire which runs
from the terminal on the magneto to the magneto switch. To "kill the
mag", the switch makes electrical contact to airframe ground near the
magneto switch, therby "shorting out" the primary circuit in the
magneto. The circuit is complete because there is a path back to the
body of the magneto though the airframe, motor mount, crankcase, etc.

When they put VHF AM radios in aircraft, it became necessary to shield
(enclose in a faraday cage) the P-lead, otherwise the very sharp
risetime pulses that appear across the magneto's Primary (resonant
inductive-capacitive) circuit, the P-lead would radiate RF energy
which would be heard on the COM and NAV receivers. btw-to be effective
in preventing RFI radiation off the P-lead, the shield enclosing it
must be grounded at both ends (to the mag body, and to the airframe
close to the mag switch).

What you are seeing is a break in the RFI shielding between the p-
lead's shield and the magneto body. This break does not prevent
"killing the mag" in the usual way, however, it could be the reason
you are hearing a "popping" noise on the weaker signals on your COM
radio.