Hollownuke

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.