Canister filter throwing air bubbles into DT

[JenniferL]

I have a Eheim canister filter for my 55gal tank. Lately once in awhile it will make a sound like it does when it first starts up and then throws a ton of micro bubbles into my DT. More so after my water change last night and I didn't even take the thing apart to clean it. ( I do that every other week) Can these filters become air locked? or is it drawing air from some where?
Also my return for my skimmer is above my intake for the filter, would it be drawing in air bubbles from that possibly and then blowing them back into my tank?
Sorry for all the questions. I just don't want to take the filter apart if I don't have to.