Ok this is driving me nuts. I have some of the symptoms but not all. I read this thread for more info:

http://www.clubxterra.org/forums/showthread.php?t=11668&highlight=knocking

but am still undecided.

I HAVE: Bad fuel mileage, rattling supercharger, somewhat rough idle, and the valve on the back of the SC pointing to the cab after a drive

I DON'T HAVE: 3000 rpm limit, dosn't feel like a lack of power.

Before I take it in to replace the knock sensor I want to be more sure that this is actually my problem. Thanks for the help!

Take a look at this

http://www.clubfrontier.org/forums/f46/s-c-frontier-knock-sensor-relocation-how2-48473/

Oh and go to Auto Zone and see if you have K/S Code.

How many miles are on your X? That "rattling supercharger" could mean a bearing failure and the guts of the supe are chewing themselves up. Are you positive the rattle is in the supe?

theres about 100000 miles (153000km and i know timing belt is due soon) I had it inspected before i bought it and they said it was the SC the rest of the engine was knock free.

*Edit* I read the relocation article, but was reading in another thread, that the sensor is where it is so it is blocked from other noises that could be percieved as knocking. Otherwise it may pick up other normal engine sounds. And does anyone know who does free scans in canada? we don't have autozone :(