Good topic !! :D
- Subaru flat-fours (Impreza, etc) are a dead giveaway - deep resonant burble
- XR2/XR3 from the 80s with the CVH 1.6 - farty
- Recent M3/M5 - they have a metallic wail, as if it's about to let go
- Bentley Conti GTs - a couple drive past my pad every day and they sound dead throaty
- Audi quattro (or indeed any of their large saloons from 80s/90s) - 5 cylinder wail
A couple more left-field -
- Any 70s-90s Mercedes truck with an OM engine - a right wailer
- Cummins truck engines - they have a signature sound, like bubbly clockwork
(my old boy used to salvage heavy trucks so was around them all my life :))