I've been noticing a vibration/humming noise comming from a 12x12 vent on the first floor of our home. The system is gas forced air and the noise only occurs when the heat has been on the last month. There was no noise this summer season when the cool air was running. It is a two-zone sysytem, first floor and second floor. I have removed the vent and the noise disipates, however is still there. I think it's the air hitting the circular sheet metal at the opening of the vent.

Any thoughts or solutions?

Sometimes we have problems with the fins of the metal vents, what I do to stop it or at least change it a bit is to use a pair of needle nose pliers and bend up each fin where it attaches to each side of the vent. This makes the metal more ridged and helps. If this is not enough bend all of the metal that you can reach inside the vent.