I have lived in a Condo for 2 years on the 2nd floor (2 story building). No one has lived in the bottom condo until recently. As soon as the bottom floor resident started using their AC I noticed that my AC would make a puffing/popping noise when not running. When my AC is running it is louder than the puffing noise so I am not really sure if it stops or I just can't hear it. Pics below: Imageshack - camera69010.jpg Imageshack - camera69011.jpg - If I remove the cap on this pipe I can feel the air puffing when the noise is happening. Our outside AC units sit directly next to each other. Any advice on this would be appreciated.