I have just tried setting output volume to 0 and that hasn't cured the problem. Just heard it while listening to EGKK_GND at 1222Z 8 May, transmission by EGKK_GND to a KLM callsign. Just heard it again as I type this at 1224Z, VIR27X calling for clearance triggered it about 5 seconds into his transmission.
I'll try disabling realistic ATC audio effects and see if that makes a difference.