I know they are too loud, but the sound of a modern 4-stroke barking and driving is superb. As a kid, I loved the sound of the TT500 and later the first couple of XR500 models. Beautiful deep notes, but without the urgency and snap of a modern 4-stroke.
But NOTHING beats a 500cc 2-stroke pulling hard through 3rd and 4th on a big open track... BRRRAAAAAWWWWWWWRRRRRPPPPPPPPP