My thought is that it is all about the length-tuned intake. The reversion pulse moving back up the intake trumpets has a sweet spot and the harmonics will create this particular tone (timbre is a perfect descriptor here). It plays off the exhaust note; which may also be length-tuned (on a race car, it should be). These tuned pipes work like a pipe organ to create a very particular harmony, and each race engine has it's own distinct version.
Here below is the e30 DTM with a race-S14 and the very particular DTM-tuned airbox. That airbox accounts for much of the sound we hear, as it thrums with the reversion through the ITBs.
Flat plane crank engines have a very particular howl in their own right, add the tuned intakes and exhaust for a racing application and, well... you get an Enzo Symphony.