Paul - this is very typical of "radio frequency interference" (RFI). When you key up the transmitter you're sending RF energy down the coax cable. While we like to think of coax cable as being completely effective at keeping all that energy inside the cable, the truth is just the opposite - braided shielding is a bit more like a fine sieve than a Faraday cage. Some of that RF energy DOES escape.
Escaped RF energy is likely coupling into adjacent wiring bundles, perhaps the bundle that contains your CHT wires.
The fix for this kind of RFI (assuming this is a newly occurring problem in an installation that's been flying trouble-free for some time) is usually a simple "assume the position" under the instrument panel and trace the coax from the COMM radio. Anywhere it comes close to another wire bundle, do your best to achieve separation. If you can alter the direction of bundles so they cross as close to perpendicular as possible that likewise will be better.
After about 15 minutes of standing on your head and judiciously applying zip ties I bet you'll find your problem is solved.
One "gotcha"... Coax cables "leak" more energy when the antenna isn't able to absorb all that energy. Do a good check on your antenna - make sure it's grounded, make sure your coax connection is clean and tight, and don't forget to ensure the centre pin of the BNC connector is actually attached to the centre conductor of the coax. This will help you rule out the antenna connection as an exacerbating factor.
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