My rev3 has been doing the same thing, as mentioned by someone else it sounds like pinking, which could be caused by the timing being out(possibly caused by wear). What I have done is remove the flywheel and rotate the stator plate clockwise by about 1 to 1.5 degrees. It has stopped the pinking and the bike feel great. Don`t forget to mark the stator plate and the casing with a small pop mark so you know where it is before you move it.