Dont really know about the Fluval type filters, but with my canister filter, it was air trapped inside the system..All i simply did was manually bleed it by slightly loosen the inlet tube that goes onto the filter for a few seconds to see if that lets any air escape, if not, i did the same on the return and i could here a quick release of air out of the system and then it was sorted..some filters do come with a bleed valve so it may be worth having a quick check over the intructions that come with it..OR..it may be that you actually have an air leak on the pumps pipe work or canister, a connection that is not seated well..
Hope this helps
Niko