Humidity and odor from furnace duct.

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HI,
I have a Coleman furnace, I believe it's original in my mobile home which is a 1985.
It runs fine, makes good heat ,etc. except this evening I noticed that there was a sizzling or crackling kind of sound coming from the duct in my bedroom after the furnace shut off.
Shortly afterward I noticed a faint smell and can feel some humidity as well. There has been a buildup of moisture on my windows for the last week or so.
Any thoughts or ideas as to why or what is happening and how to fix it?
Thanks for the help.

Bpark

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Could be condensation or some kind of a water leak. Get a mirror and flash lite and take a look. I do not believe what you are hearing is normal but I have no way of telling you what it is from here. Take a look.

hvac1000