Do you have a sdr to try operating in car to try and find the noise source??
you try a external battery??
Might try any of the sound modems on extra pc or raspberry, might also help your situation, but at minimal having a second tnc around helps diagnose the first! You can plug in a laptop mic to the speaker of the radio for example to have two tnc devices quickly to test with (could even use the same radio!)
kelly?
