You are experiencing radio-frequency-interference (RFI) being radiated
from a P-lead directly to the COM antenna, or RFI radiated from one or
more of the shielded HV ignition harness leads running from the mag to
the spark plugs.

The ICOM sees the RFI noise pulses at its antenna input, and thinks it
is a valid signal and opens the audio squelch, so you hear it. If the
ICOM had an manual squech control, you could probably just set the
squelch tighter...

There are coaxial RFI bypass capacitors built in to the mags. They are
the first line of defense in suppressing P-lead radiation. The P-lead
running from the stud on the mag through the firewall to the Mag Switch
must be shielded (unbroken), and the P-lead shield MUST be connected to
the body of the mag at one end, and to airframe ground under the panel
near the Mag switch at the other. Rebuilding the mags with new
capacitors may help.

The HV spark leads must have a continuous shield from mag to plug. The
plug end gland nut is aluminum, and sometimes it makes a bad connection
to the outside of the plug. The other end is bonded to the shield over
the HV tower inside the mag. The usual cure for this problem is a new
set of ignition harnesses.

Do not screw with putting external capacitors in the P-leads. The
internal mag capacitors form a series-resonant circuit with the coil
primary, just like an old point and condensor ignitions of old (up to
1960s cars). Putting external capacitors (especially the ones you are
talking about) will greatly degrade the spark energy.