I can hear water drops on wood (one drop in about a minute) in the dry wall when furnace is working, and even in the first few minutes after furnace stops. Last week, I had a person to perform furnace maintenance and asked him this question. He said that was normal, still, it causes my concern. The only thing I can think of is moisture condense on some pipe and then drop down from pipe. Even that, I feel it is not normal to have water drop down.
Your advice will be sincerely appreciated.
Thanks.
andrew.