I noticed last week that when the car is at idle,i get a strange humming noise from underneath or the engine bay. also noticed if i accelerate it stops. Another thing is my ac has to be charged but if I turn it on it goes away sometimes? Can the switch be bad? Anyone come across the same issue?

 

Sounds like you may just have something loose with the exhaust , a heat shield or just exhaust pipe hitting under the car making it sound like it was coming from the engine compartment. Especially if you changed the exhaust you have a bigger chance of noise. Best thing to do is get a buddy and have him/her either sit the car with it in gear or running and you look around the car and pinpoint were the noise is coming from.

 

Are you sure its not coming from the back of the car? My fuel pump makes a humming noise when I get low on fuel.

 

I figured it out, it was a vacuum leak. The rubber T fitting coming off the PCV valve split in half..