I've always enjoyed the combined engine/airframe noise from a Centaurus powered Sea Fury during a fast flythrough. Unfortunately whenever I hear a Merlin some dirt seems to get in my eye!
Not just pure engine noise and as other have already mentioned, Vulcan howl, Hunter blue note, Goblin whistle and Olympus 593 crackle all grab your attention, but for me the most impressive noise ever was produced by the massed take off of Lightnings at Farnborough in the 1960s - I've heard nothing like it since.