ECB wrote:

Any ideas? Thanks in advance.

You might first try switching to one mag & then the other to see which
one is making the noise.

Then - is the noise that of a popping at a single cylinder firing
frequency (which would be a bad plug lead as described by others), or
is it at the much faster engine firing frequency - which would suggest
a bad magneto capacitor or p-lead (i. e. the wire to the ignition
switch) filter. There is a remote chance the p-lead shield(s) could be
broken.

It probably is not an engine ground problem - at least not if you don't
have starting problems, or alternator noise or regulation problems.