It sounds like it has air in it. Try pumping the priming button a couple times and "gently" rock the canister back and forth. If a lot of air comes out and it's still making noise you need to remove the canister, empty the water, the rehook it up the same way you would after you clean it. Then it will usually prime itself just fine and the rattling stops. There is one other thing... I've had small snails, especially Malaysian Trumpet Snails with their harder shells, get up in the motor and cause problems.