The reason that you hear the '-r-r-r' noise is due to the high bypass ratio fan (N1) being at the front of the engine. This fan is turned by the power section(N2) of the engine and in turn "sucks" more air into the inlet the faster it turns. This is known as a "tractor fan", or air pulling unit, thus the N1 fan blades making the noise you hear. Most all high bypass fan engines make this noise to some extent.
The other reason that you can not hear the noise from the rear is the reason why these engines are Stage 2 certified. The N1 section of the fan moves high velocity air around the engine nacelle and also around the exhaust. This quietens the exhaust and thus makes the noise footprint smaller and almost indetectable, hence a quieter engine.
Hope this explains some of the phenomenon for ya...cheers
Airbuswhiz