DD Pump "Thumping"/"Pulsing"

Hello fellow overclockers, I just bought a Danger Den DD-CPX1. during the process of the bleed, it started thumping when I turn on the box it's attached to. It flows for about one second, and then it starts to "thump". I'm totally lost here. For a while before this started happening, it would start for a second and then stop.

While it's doing this, if I pinch the hoses in front and back of the pump it starts to go, but sounds like an old hard drive on a torture test. Is this just from bubbles? Any clues are appreciated!

Here's a picture of the pump. The power is connected to a 3 pin fan connector in an old computer next to it. Is it bad for the pump to turn it off and on rapidly (off then on again in about 1 second)? Thanks for the help!

It's in mid air.... isnt that a problem.

The impeller might be knocking back and forth.Take th epump out of ht eloop, open it up and make sure the impeller is sitting in the housing correctly.

Depending on the style of th eimpeller yo umake here it "rattle or thump" as air goes through it. That causes cavatation, and the impeller may shift back and forth.