Sound I most love is the pop and crackle of a Merlin when the throttle is closed on landing. The airframe an engine is installed in contributes to the sound as well, the P-51 Merlin in flight sounds better than a Spitfire, R-1820 in a T-28 or S-2 sounds far better than a DC-3 or B-17, but all radials sound great. Turbines don't do it for me, save the growl of some heavies on take off.