On Mon, 10 Jan 2005 14:48:27 -0700, Andy Hill
I seriously doubt there is any sort of air hookup from the furnace output to
your vent stack. Seems a lot more likely that one of your water pipes, or the
drain stack / vent stack for that sink is now contacting something in your HVAC
system. You sure there is no "vibration" felt in either of the water lines
or the drain pipe? I'd think tracing the vibration back would be your only
option.
I didn't feel a vibration beneath where the drain pipe is and I could
definitely feel air coming from the drain.

Is any type of air vent connected to the plumbing?

- Samiel