It's the transmission's noise.....It comes on after about two yrs or so on RX8s....it can get a bit louder as the car gets older.... it's pretty normal...


But...you can try changing your transmission oil.....In my case, it went away after I replaced my trans oil (atleast it's quieter) on my 04.....it's been about six months now...no noise whatsoever...I don't hear anything in the cabin..... But I'm sure it will come back eventually...lol ...Atleast thats what I was told

 

The gearshift kinda purrs in your hand at idle. It's a side effect of being connected directly to the inside of the transmission on the other end.