96 Villager Humming Noises
 

I have a 96 Murcury Villager to which about a month or 2 ago started to hum. When I idle it is a steady hum. When i excellerate it gets increasingly faster, but when i let off the gas the numming goes down as well. I stopped by Auto Zone to pick up some anti-freeze and asked the person there. They said it could be a couple different compressors. When i got home I added a little refridgerant into the A/C system and the idleing humming was still there but not as bad. Can someone please give my some advice as to what I should do next. Thank you in advance.