The most amazing mechanical sounds I've ever heard were at the high-bank oval at Daytona during the 24 Hour race. There's something about how the oval just holds in the sounds. You just don't get the massive reverb at Sebring or Road Atlanta like you do at a super speedway.
Anyway, over the years I've heard shrieking, silk-tearing Ferrari V12s, thunderous Cosworth, Ford and GM V8s that induce a sympathic vibration right in your diaphram, jet engine 911 turbo GT2s and blatty straight six M3s.
For me, nothing really comes close to a tuned NA race engine. Turbos are all and good; the Audi R8 sounds really cool when it shoots by... IMO, NA motors make the best noises. I can close my eyes and still remember the 333SPs and Ford powered R&S Mk IIIs and the C5-Rs...
M