A/C Sucking Air

Ok well when I am driving down the highway and have the A/C running or just air blowing out and I give it quite a bit of gas for example going up a hill it starts to suck air in and quits blowing it out. At this point it sometimes makes a light knocking noise, but if I let off the gas the sucking goes away and the noise does to. Does anyone have any ideas on what is causing this? Thanks for all your help.

Yep. Vacuum leak to the heater/AC controls. Usually caused by a broken vac reservoir behind the passenger side bumper or a bad vac hose between it and passenger side firewall..

Ok I will check it out when i get a chance and let you know if it fixed my problem. Don't know why I didn't think of that.