Whenever we shower or use any appliance in the two adjacent bathrooms we hear a consistent sound that comes from a vent pipe diretly over head the bathroom that sounds like a drip. The drip intervals are very consistent at about 10-15 seconds apart. We will hear the sound for about 15-30 minutes after use of the sink, water closet, or shower. Extremely annoying when trying to sleep.

I replaced the ceiling and found no evidence of water damage.
I have identified the spot in the vent pipe, along a horizontal run, that the sound eminates.

I do have access to this pipe as it is in the attic.

Any ideas as to what causes this sound and how to eliminate it.