F1 engine double sound?

[geekdiggy]

i know i'm not the only one who notices this. k, F1 engines are known for their high revs and therefore very high engine note, very whiney almost. but everytime i watch i also hear a lower note, tangent to the higher not but a couple of octaves lower. i wanna know, how come the F1 engine makes two different notes both low and high?