Radio Static When Engine Runs

Fellow Flying Dudes,

I own a '63 Piper Cherokee with King KX 155s ... the radios both run
fine .... until I fire up the engine. There is a lot of popping and
static that changes a bit with adjustments in engine speed. I have
turned off all other equipment to omit other causes. I have shut down
the alternator as well via the field and armature breakers.

A fellow flyer said it could be a bad/intermittent ground between the
engine and the airframe.

Any ideas? Thanks in advance.

Ed B - N7025W - MMU