The bass slurping on the surface is interesting and may indicate feed trained fish. It has now been a year since they were fed, but they may pick up on it if you begin a new feeding program. I am not the guy to talk about feeding though. Theo and others have far more knowledge in that area.
Couldn't this "slurping" also be the bass "piping" as in low DO in the pond? Have you checked the pond's DO during the observance of this "slurping"? I don't know, I'm no expert, more like one of the village idiots, but I seem to recall reading that fishing piping, slurping, gulping, or singing Barry Manilow songs while at the surface of water is not a good sign.
