I've had two xp2's that ran perfectly for years, then one started gurgling and burping. turns out my goldies had pushed their bubble disk under the intake and it was pulling air into the cannister. I moved the bubbler away from the intake (and anchored it down so the little brats can't move it again!) and it hasn't made a single sound since. My xp3 seems to be rather picky about how I re-connect the quick disconnect valve after cleaning. I have to push it firmly straight down, give it the slightest little wiggle, then one more straight down nudge while closing the latch. If I don't do it "just so", the cannister doesn't fill completely to the top with water and every 15 minutes or so it will dischrge a gush of air into the tank. When I get it right, it's perfect. Good luck-I love these filters, quirks and all!