Air rises up through grainbed when I start to drain into BK

[Smellyglove]

This is a puzzling thing to me.

Sometimes (like now), when I start to drain to the BK, large amounts of air rises up through the mash bed, and basically destroys it. Grains just starts to float around, quite dense, and I get pretty cloudy wort into the BK.

I'm using an Infussion MT and a march pump, I do no sparge. It happens when swap the exit-hose from the march from my HEX, to the BK and start the pump. Huge amounts of air are released.

There is no air inside the pump when mashing. It must come from somewhere. Can it be trapped under the FB without being sucked into the pump, and for some reason release? Maybe I'm having a negative pressure under the FB, which equalizes when I disconnect the hose, thus releasing the air, or something?
I pump to the BK with a slower speed than when im recirculating.

 

[lschiavo]

It sounds like you are sucking a cavity under the false bottom. You may have to dial the pump back a bit when you recirculate. Are you seeing a continuous flow at the exit of the hex?

 

[Smellyglove]

It sounds like you are sucking a cavity under the false bottom. You may have to dial the pump back a bit when you recirculate. Are you seeing a continuous flow at the exit of the hex?

I belive this might be the case. It's continious, but some times it gets lower, so I dial up the pump quite drastically for just a half second to get the flow going again and dial it back down again. Then air exits the outlet from the hex inside the MT. I usually start slow. And I do this often just in the first 1/4 of the total mash time.