We hear a dripping nosie from the vents at night when the heater is off it is about every 5 to 30 seconds. I checked the vent pipe and found no signs of water. Can you please help it is keep of my kids

No caps needed, we can still read fine print. I believe you are hearing the ductwork contracting as it cools. It expands when heating and you don't hear it because of the fan noise. When the fan goes off, the duct cools and makes "dripping" noises. More noticeable in attic ductwork.