[BTCC] Question on supertouring/S2000 engines

[Sodemo]

Ever since I started to go to touring car races i've always loved the sound of the naturally aspirated cars. The cars seem to have two distinctive sounds. One is a "buzzing/growling" engine note, and the second is an exhaust note.

A video to illustrate...
http://www.youtube.com/watch?v=cg-VYT01vnk
0:16-0:22
1:16-1:19
At the above timecodes you can hear a very distinctive engine type noise (there are more throughout the video, I have just listed these above as two examples). It seems as though these cars make this noise as they are moving towards you, or the camera, and when they are moving away from you or a camera, this noise becomes the exhaust note.

Another video...
http://www.youtube.com/watch?v=1Lt1PkMZrCM
0:28-0:33

I guess my question is really; what is done to these engines to make them sound like this. Is it a specific part which is changed to give it this growling/buzzing sound?

I hope people don't think im mad, and actually understand what i'm on about.

Many thanks,

Ben.