could also be the pressure switch on the pump isn't any good. They can be fixed, but might be easier to get a new pump. But any small little leak will cause the pump to pulsate, more often than not with a small leak it will turn on, then turn off, then turn on. It doesn't constantly pulse.
It also could be there is air in the lines. I had a similar issue, and then after I drove it, the pulsating noise quit the next time I turned the pump on. I was about ready to replace the pump. Even bought a new one. Maybe driving it cleared out the air. Not sure.