Assuming your pump is not rubbing against anything, Vibrating pumps are usually loud due to back pressure. You need to use a gang valve which has at least 1 more valve than you are currently needing. Start with all valves closed. Open up all the valves connected to your objects to slightly higher levels than you want to run then slowly open up the last or unused valve to let the air escape. Open this valve until the pressure to your other objects gets to the levels you want. ( some readjusting may be necessary.) This should not only quiet down the pump but make the pump run more effeciently and make the diaphragms and flappers inside the pump last longer. Try NOT putting anything under the pump as this is where most pumps have their air intakes. If you block those, the noise will be worse.
IF the noise coming from the open valve is too loud for you ( there will be some hissing depending on how open you needed it to be), you can attach a piece of airline tubing and an air stone to that last valve to act as a buffer. Do not put this in the water however. This is just to baffle the sound.
IF this does not remedy your situation, there is a good chance you have too large a pump for what you are using it for. It would then be recommended that you downsize the air pump.
Hope this helps
