Have you tried just jiggling it around to get it in a slightly different position? Or perhaps you need to take it out and clean it. One thing you could always use as a last resort is "greasing" it with a tiny bit of vaseline.
On my hydroponics I cut a small piece of airline tubing and put it behing the washer by the impellar, silent as any thing..........havn't needed to do that to the tanks yet I think the small pink fish is a dwarf Gourami, could b wrong though