Dripping sound on basement ceiling

My finished basement ceiling has intermittent sound like water dripping on a piece of metal occurs most of the time (not all the time). It is annoying at night

I cannot think of any cause, around the ceiling area there is only a heat source from high efficiency furnace coming down from ceiling

No sign of leakage. Sound comes from 2 different spots on ceiling. I cannot open up the ceiling to check it because the ceiling is stucco finished.

It happens anytime even when I do not use toilet/faucet. I close all toilet, sink, faucet water valves upstairs but the issue still persist

Has anyone experienced this issue or know the cause of it?
Asked Dec 21, 2015

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