Transmittion Noise - High Power Settings

We recently upgraded the avionics and instrument panel, which was a great
improvement. However, I'm having a problem transmitting under high
power. I get a crackly static (during transmittion only) when over say
20" MP and 2500 RPM or 25" MP and 2200 RPM. At the start of the
attempted transmittion there is a lot of static and then if I hold the
transmit button down the static slowly dies away after about 5 to 10
seconds but the transmittion is still garbled (it is not sync'd with the
engine RPM - it is either there or it isn't). With less than those power
settings the transmittion is totally clear with no crackling. All during
my recent IFR training we never had the problem, however, my instructor
was good about taking it easy on the engine and other than climbing we
were always at 20" MP and 2400 RPM or less power. I'm thinking we would
have noticed the problem under higher power settings and the problem
would have been intermittent.

I had this problem once in a while even before we did the avionics
upgrade. It again started about 5 hours ago and is now a persistent
problem that I need to get fixed. Which leads me to believe that there
is something else going on besides anything with the avionics. A
vibration issue that adversely impacts something? My instructor and I
both didn't notice any unusually bad vibration at high power.

This is the troubleshooting I have done so far:

1. Determined the power setting where the problem starts to happen
(over say 20" MP and 2500 RPM or 25" MP and 2200 RPM).

2. Shut off each mag and the problem persists (eliminates the mags?)

3. Shut down the audio panel to fail safe problem persists
(eliminates audio panel?)

4. Problem exists on both COM1 and COM2 (eliminates the antennas?)

5. Swapped COM1 and COM2 in the trays and problem persists
(eliminates a bad radio?)

6. Tried transmitting from both the pilot and copilot positions and
problem persists (eliminates microphone wiring and headsets?).

7. Cleaned up and fixed up the P-Leads on both Mags (they were in bad
shape and now even though they are the old wires, the shield connection
is good) - still have the problem.

8. Tightened the engine-grounding strap that was loose at the
firewall bolt. No change in the problem.

9. I purchased two Mag filters, however with my mags I cannot find a
good place to mount the filters so I have not installed them. Seems like
a band-aid as the problem was intermittent and is now persistent and I
would rather find the problem and fix it.

Other things I have not tried but I'm guessing could be the culprit is
loose spark plug wires, generator or maybe the voltage regulator -
however 20" MP and 2500" there is no problem so I'm not leaning towards
the generator as the problem.

I really need to get this fixed as there is no way I can blast off IFR in
the fog climbing and talk to ATC.

Thanks for anything you can suggest. Sorry for the novel. I just wanted
to make sure you had all the details.

Paul Smedshammer
1967 Mooney M20F, N3506X

Equipment: PMA6000MC Audio Panel, Narco COM1 810+, Narco COM2 810,
Nav1/DME/RNAV KNS-80, Nav2 Narco 825. Not state of the art but it works
great - other than the transmittion problem!