For now, I wasn't going to put a drain in. The run is so short, like a
three feet, that I doubt it will help much. The drain on the gun
sounds like the way to go.

Likely you
also received rubber pads for isolation with your compressor. If not, get
some 1/4" rubber about 6"X6" and put them under the legs of your compressor.
You'll be amazed at how that quiets down the hammering that transfers
through the compressor to the floor and rings within the tank.

Not only did it not include them, the manual has some very specific
instructions for bolting it to the floor complete with drawings, type
of expansion bolt for the concrete, and torque specs for the lag
screws. The manual says that if you don't bolt it to the floor,
excessive vibration will cause the tank to weaken and possibly explode.
This sort of things frightens and annoys me. It sounds like they took
some engineering shortcuts and passed the problem on to the user.
Still, the ringing tank sounds like exactly the type of vibration that
would cause metal fatigue or whatever weaking they're talking about.
Maybe a call to porter cable tech support is in order.

brian