I had a similar issue with one of my m60's back when I was crazy about treating my room with acoustic panels. When using the RealTraps test tone CD that goes from 10hz to 300hz in 1hz increments (takes a long time), I was hearing this weird buzzing sound. At first I thought it was just some weird relective thing with my room.
However, after closer examination as you mentioned, I could really hear it if I placed my ears near the back of the speaker. In my situation, it was the loudest if I placed my ears down by the speaker terminals.
I also noticed by pushing on the terminals with my fingers, I could either stop the noise, or change the pitch of the noise. I then checked to see that the four screws holding the plate in place where snug, they were.
Sometimes air can escape and create a buzzing sound. This applies to the drivers as well. The next thing I tried was to start loosening the screws to check inside where the crossover is located. As I was loosening the screws I noticed the pitch of the noise changed, eventually with the right combination of tightening some and slightly loosening the others the noise stopped.
I did report this to axiom and they sent me a rubber like gasket to place behind the plastic housing, have not had a single problem since. Oh yeah, I don't own them anymore, I've go 80v2's now.
