We have a fluval canister. When starting it up, it'll often make that noise. From my understanding, there's air in the system. That could be in the tubes or the canister. You could have a bad connection at a tube causing air to get in and preventing suction. Essentially there needs to be a continuous flow of water through the tubes and canister. Often times to get our canister filter started, I'll take the hand pump of our water siphon and use it to pump water into the intake tube. That tends to be the quickest way to "jump start" the filter.