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.