I wish I could say that I did something clever there... but mine does that too. It would probably start scare your dog if you really let the speed get up there. I just played with the multiplier between the speed and the pitch of the engine. Really I should set that up so that the pitch of the engine is driven by the throttle and not the speed, so that it is user control dependent and then clamp that top and bottom values so that it can't get out of a pleasant audible range.
by the way the sound is actually a recording of a fridge compressor.