Old thread, but I want to spread the word about my solution to this problem.

I had a sound in my hvac duct that I swore was water dripping. It sounded exactly like water. But it wasn't a drip at all. Instead it was, as many others suggested, thermal expansion. But it was a specific form of thermal expansion. Specifically, my water drip sound (I swore it was water!) came from the point where an elbow HVAC joint joined with my long, straight HVAC duct. I put a large worm gear clamp around the point where the long, straight duct hit the point where it joined the other duct and tightened it down hard. That worked like a charm, the sound immediately went away. You can test it before you use (or buy) the worm gear clamp by grabbing the duct with your hands and squeezing while it's making the sound. If it stops, buy the band and tighten it right where the two ducts join. Mine hasn't come back after a month or two of putting the clamp on. Just google "worm gear clamp" and you'll see what I mean. Just get a big enough one for your duct. Good luck!