We were talking about this on one of my threads last August. The shop gave moondoggie another cap with a hole in it and that cured the problem. He said it feels like its sucking air in when he puts his hand there.
I've tried removing the cap to see if it was a vent issue, didn't seem help my saws at least. If he feeling vacuum buy the cap, or the holes in the cover. I assume the cover, in that case the air injection sounds weak.