air conditioner loud hum on high setting???

I got my '09 Tacoma back from the dealership last week after 30K service and noticed this weekend that when the air conditioner is on high, there is a loud hum sound. That sound was not there before. Can someone tell me what's causing this and if I can fix it without having to go back to the dealership?
Please help. Thanks in advance!

 

It might be the blower motor. I had a problem with mine as well. It was more of a ticking sound though and I've heard that the past few year taco's have been having problems with the blower motors.

 

This might be too simple, but last week my truck was making a loud hum/whistle noise when the A/C was on high. I was searching the cab for the problem and playing with the nob settings when I opened the door to go under the hood the noise went away, shut the dorr and the sound was back. It turns out one of the windows was just barely open and it was the sound of air escaping the cab through the cracked window.

 

check your blower wheel and make sure there isn't any trash in it- pull the glove box and cabin air filter and inspect.