I also suspect a ground loop, although a ground loop is more common in a
mobile installation. The common mode modulation is also common in direct
conversion and regen receivers. In those cases, it is solved by rf chokes
in the power supply circuit, isolating the power lines from any rf getting
back into the power supply. Hopefully, there is no real problem, but if
there is, isolate the power line from rf - perhaps both on the receiver and
transmitter.

And, it seems kind of early to check the equipment out for Field Day, isn't
it? That is something you are supposed to start Friday night and then have
to stay up all night to fix it, so you can start Field Day bleary eyed.
Seems almost unfair to check this stuff out now.

73, Colin K7FM