Sometimes air pumps will make noise when the air flow is too restricted. For some reason, air stones and output tubes that go into a filter become clogged. You can clear the clog by blowing on the input end of the air tube. Failing that, there is the more hazardous step of sucking a little bit of water into the tube then vigorously blowing it out. If you become too enthusiastic or careless about about the intake part, you can easily get a mouthful of aquarium bottom water cooties, and no amount of mouthwash will help. As an aged aquarist who has started many a syphon the old fashioned way, I can tell you that an aquarium bottom water cootie cocktail will give you a terminal case of MTS (multiple tank syndrome) but usually nothing worse.
But back to the air pump. If clearing the airways has no positive effect, try to open or close some of the valves to adjust the back pressure. Putting a mouse pad under the pump could also work.