Had that on my rp2 and a DHX 5.0. It has to do with oil being sucked into a different location than where it was supposed to be. A simple air sleeve service should fix it, but I wouldn't worry about it. I was told it causes no harm, and after riding the shock that way for a while, I never noticed any problems.
I dunno, I had a Float shock on a 2001 Stumpy FSR that made sucking noises and (luckily) was rebuilt to fix the IFP by Fox under warranty. Sucking noises are more likely to come from the damper than the air can, I guess unless there is a lot of oil in the air can.