Thanks Shaman, appears to be the same. No chance I'll RMA and spend another 2 weeks without a PC ha ha - besides, everyone's RMA replacement seems to be having the same issue so RMA would be pointless. I hope EVGA is right and it's a driver issue. Meanwhile I might try a custom fan profile:
40C=50% 50C=65% 60C=75% 65C=85% 70C=100% (curtesy of Sean1976)
Edit 1: custom profile made no difference: it sounds like the fans are revving up and down all the time, however no fan speed indicator indicates this. Sounds like a howling wind in a room, if that makes sense?
Edit 2: turning off Precision removes the revving issue completely (good!), the 'howling like the wind', hoever persists, albeit quieter.