Buzzing noise only at around 2,000 RPM

My maxima makes a buzzing noise at about 2,000 RPM. My SES light has recently come on and the codes read as follows:
P0755- shift solenoid B malfunction
P0328- Knock sensor high input bank
P1456- Auxillary emissions controls

I am very worried about all of these and am trying to do as much research as possible.
My car runs and drives. The transmission does not seem to shift hard, clunk etc. My mechanic seems to think that there may be a wire shorting or something small causing all these codes to pop up. We are getting under the vehicle tomorrow. Also, for the P1456 code I am thinking the fuel filler cap? My gas gauge seems to bounce around a bit and I do not quite trust its accuracy.