If it's a ground fault then it's more likely to exhibit problems on Tx when the current draw is higher. Same with a bad antenna - high VSWR will always make the transmitter worse before you notice much effect on rx. Typical VSWR on a general aviation VHF antenna should be better than 2.5:1 at end of the band and ideally less than 1.5:1 at around 127Mhz. The fault could be a duff screen or filter on a magneto P lead. In any case, if the fault can be reproduced on the ground with engine running, it should be isolatable by turning each item of electrical equipment off during a ground run whilst having the squelch open and listening to the noise on a weak distant station ( a distant volmet transmitter is good for this). If the fault is only apparent in flight, look for poor bonding of a flying control or undercarriage door or poor static dischargers.