757 sound...
 

I posted video earlier of a low approach (cleverly titled not standard departure)... but anyways... wondering if anyone could answer to why the Rolls Royce Rb211 engines sound so amazing and different?

Because the Rolls 757 is amazing and different.
I've flown the 757 with Rolls engines.
I'm not sure why it sounds different.
It is a 3-spool engine vs. 2-spool.
The exhaust is very different from a Pratt or CFM.
These may be why, but what do I know?

The above is pretty much all I can figure out. I'd say the 3-spool design is one of the biggest factors, parts are all moving at different speeds producing different sounds etc...