Suspend the air pump from the inside of the stand with a piece of string. The vibrations from the pump are made louder by resting on the stand, the same as if you take a tuning fork, smack it, then place it on a table.
If you get a valve to adjust the air pressure you can turn down the air flow, making the bubbling sound quieter. For now tie a loose knot in the air feed line.
With any air pump it's better to bleed off some air than to turn down the output with no place for the air to go, but for a day or two reducing it with a knot won't cause any long term damage.
