with the heater on an defroster set to the windsheild I will hear a hum once or twice a minute. I also notice a very slight decrease in the rpm's. what could be causing this?

 

Defrost mode cycles the AC, hence the rpm drop, the hum could be a vacuum servo leaking or making noise. unhook supply line from the vacuum pump and hook a hand held vacuum pump to it, pump it up with the defrost selected and see if it holds vacuum

 

pop your hood and see if your a/c compressor is cycling on and off. Mine was doing that before when I had it in the vent mode. I still don't know why but it has quit and the problem never returned.

 

Funny, looks like Nick just answered my question too.