The whirring sound you get when braking on some nicely warmed up rotors on a really fast road descent.
The tiny electronic zzt of a Di2 front mech self adjusting.
The sound of nature when you properly get away from it all, stop and all you can hear is a cuckoo or the curlews or an owl.
The sound of a proper elite peloton.
I’ll add to that one, the sound of a true pro DH rider on the limit during a run – it’s a soft rumble then a whoosh as they go past. Compare that to an amateur where everything is creaking and clattering and banging and squeaking.