Woodsman...did you add Dynamat or something similar behind the seats and to the engine cover? I searched a bit and didn't seem to find anything here on sound deadening. I found a link on P-9, but they were more concerned with exhaust drone.
Dynamat installed, w/ photos
I find that doing a lot of winter driving (i.e. top up) makes me (well, really my S.O.) notice the slight higher-pitched whine of the engine, so I was thinking of adding a layer of Dynamat Xtreme and some Dynaliner on top.