My pump recently started doing this. In my case it turned out to be the drain plug on the pump itself. I had lost the o-ring on it, and forgot to add some RTV sealant before starting everything up. Worked fine without bubbles until the added suction (and a little cavitation) when the vacuuming started. Easiest way to find something like that is cycle the pump on and off. When you cycle it off, water will tend to "back out" of any leaks that you might have. Sure enough, I watched a couple drops of water come out the drain plug on my pump.

Another possibility is the cuff for your vacuum hose. The swivel cuff is meant for the vacuum head itself, as it is not sealed. Make sure you have the swivel end attached to the vacuum. Its also possible that even the sealed cuff has become a little less sealed and is sucking air while vacuuming.....my guess is you would hear that though.

Try the pump cycle option to see if you see any water dripping. Then Id try to see if you can get a known good vacuum hose after verifying all of the connections on the current one are correct.

-Chris