Clean it first and use some fish safe silicone lubricant on the metal shaft. Lube up the whole metal shaft with a wooden skewer or anything that will let you reach in there or take the metal shaft out and grease/lube it up. Slide the impeller up and down on the metal shaft for a bit to get it greased/lubed up. The silicone lube is normally used for greasing/lubing O rings in canisters and stuff and can probably be bought at your LFS and guaranteed online. Worked on my new Hagen HOB's: Fluval C4's and Aquaclear 110 (previous Marineland fan, wanted to see how great these Hagen HOB's everyone raves about). When I got them they had a slight clicking/rattling noise. Now they run incredibly smooth and quiet. I've heard of people using vaseline but it could eventually wear out rubber parts in your Aquaclear (maybe not but i'm not gonna risk it).