I had a similar problem, but mine did not pulsate on near that often. Still very annoying. I replaced the pump. But after I got the old one out, I discovered 2 screws holding the pump mechanism to the motor had come loose and were missing. I found one on the floor near the pump, the other must have bounced out where the pipes come through the floor. And the other 2 screws were loose.
I replaced since I bought a new one anyway, and keeping the old one for a spare now. Hopefully will never need to use it. Still, simple check. Make sure all the screws on the actual pump are tight. It could just be sucking air because of a loose screw.