Often times the pump will often disrupt the botom of the pit and "stir the pot" so to speak. Couple of options:
Clean the bottom of the pit and drop in some 1" clean rock and place pump on top of that.
Get a small patio block and place the pump on that. This will keep the pump from stirring the bottom of the pit, and also you'll be able to tell if the particulate is actually coming from elsewhere.
"Only after the last tree has been cut down... Only after the last river has been poisoned… Only after the last fish has been caught, only then will you find that money cannot be eaten." Cree Indian Prophecy.