Im not saying this is the best sounding rs but when I owned my old rst cab it had the same grumble as this,very similar to a subaru,I dont rev mine like this but when mines reved it sounds like a 3i till it comes on boost whats the reason for this,

http://www.youtube.com/watch?v=ZBhD_sIZBzU

 

It's all to do with the exhaust system and how much boost your running I think

 

The rs rumble comes from the std un-even manifold IIRC.

I guess it can vary on the make of system you have and the size of the tail pipe.