We just bought a house that is only 2 years old with Trane Dual Heat Pumps. Just this morning, we turned on the heat (it had been shut off for the night) and it starting making this hissing noise very loudly out of one vent. I walked in the other rooms and I can't really hear it - it sounds like it is coming out of one vent only.

Any ideas of what it could be?

Thanks!
James