First off, just hook up some airline (the longer the quieter) to the other outlet. (As mentioned above)(Or buy a Y adapter, and put both outputs to the one airstone you have, you'd have a bubble volcano then!!)
To lessen the humming, put the air pump on something that'll dampen the vibrations. I think people put it on little carpet squares... or I dunno, sometimes putting something heavy on top of it quiets it a bit (depending on what it's sitting on).
An airstone really isn't needed if you have enough surface agitation. Many people argue that airstones don't even put that much Oxygen back in the water anyway. I didn't run an airstone on mine before I planted it, and my fish were fine.
If you like the look of the bubbles, keep them. It can't hurt anything, some fish even like the bubbles. I know theres a vid on here of somebody's fish hanging out in the bubbles, gettin a bubble massage.