How are you switching it from stby to receive? If you're switching the B+, that's a lusing way of doing it. The PS voltages raise up plus it's really dangerous. Ask me how I know this
Best way is to open the RF gain pot's path to ground and switch that. Shouldn't be too big of a "pop" when you switch to receive, if you put a 47K resistor in there to keep a bit of DC current flow going.
Also could be a relay sequencing issue. If the transmitter's still "on" during the transition from TX to RX, you'll get a big thump due to the strong signal the receiver sees. Generally this isn't a problem if the coass relay is switched from the xmitter. The xmitter relay turns off, kills the xmitter, and the coass relay (because it has a short inherent mechanical delay) switches the antenner and the RX from stdby.
Which begs the question, are you using the aux contacts on the coass relay to switch the RX to stdby, or some other method?