As the previous poster says - it sounds like it is sucking air in to the pump on the inlet hose or connector to the pump.
It sometimes helps priming the pump if you remove the high pressure hose from the pump outlet. Then once the pump is primed, reconnect.
It is best if you are gravity feeding, as you can run the water through the inlet hose to remove all the air before connecting to the pump.
If any of the valves are worn or sticking it will make it difficult to prime, you may also notice a pulsating of the lance and high pressure hose when in use.
Most decent triplex pumps will draw water from a barrel. It is best to use min. 25mm inside diameter water feed hose from barrel to pump to avoid flow restrictions.