You probly have an air bubble. It may be trapped and you could work it out, or you may have a very small hairline crack in the top so it can't form an airtight seal.
I've had a Rena XP3 for a couple of years and never had any type of issue. So when they came out with the XP4 a few months ago I snatched it up. I spent litterally all day fighting with it. Tilting it, shakeing it, bumping it. Get the bubbles out, and 5 minutes later, here they are again. After fighting with it from 9:30 in the morning until after 11 that night, I gave up and put my old XP3 back on. No problem. The next day I took the XP4 back to the store where I got it from and told them what was going on. They checked it out and found a very small hairline crack where the intake hose attaches to the canister and the black rubber seal had a knick in it. They told my that is what the problem was and asked if I wanted to exchange it. Heck no, they don't have a clue how much work it is to take out a filter and hook out another system, just to take it off a few hours later. I wasn't going to spend a second day fighting with a filter.
I'm not saying one is better than the other, I don't know, I've never used a fluval. I am saying, check the area around the intake and return hoses, check your rubber seal. Something must be off and lettting air into your system. These filters should not burp and blow bubbles.