Sound of water drops when furnace is on

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.
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Is this a late model efficient furnace? They have drainage systems for condensation. Does yours have a condensate pump or is it high and have a condensate drain? Maybe it is plugged and condensate is backing up though usually if that happens the furnace shuts down. Hard to know what is going on without seeing the instulation.

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Thank you, Art for your reply.
Well, I am not sure if it is efficient model since the furnace is the original one and the house is 26 years old. I live in a town house, and furnace is in the basement (on the ground) and there are three floors (including basement). The water drop can be heard from the second floor (living room), and from vent outlet in both bathrooms of 2nd and 3rd floors. From vent outlets, it sound like the water drops on metal. but in living room, it sounds like dropping on wood on the top of 2nd floor. The chimney has double metal layers. According to the maintenance person, there is no water in the chimney right out of furnace.

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That old it is not energy efficient and the condensate probably goes up thru the chimney with lots of hot air. Honestly, from this long distance there is no way to know if you have a problem.

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On Sun, 28 Nov 2004 23:07:59 -0500, "Steve@carolinabreezehvac"

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