I have a problem that has me confused. When I turn on the hot water handle on my bathroom sink faucet I hear a dripping sound. I have investigate the hose from the faucet to the valve and found no moisture. I have also checked the valve up to the wall and see no moisture. No wetness on the cabinet floor either. No leakage from behind the faucet on the topside either. I have double sinks and do not hear the sound when the right hand sink is running. Only from the hot on the left handside. I am confused because it does not make the sound when the faucet is turned off. Of course I only hear it when it is quiet in the house. My house is 12 years old and the sinks and the faucets we changed out about 3 years ago. It is a definite drip sound not like pipes expanding. The sinks are also located on the inside wall of the house so the pipes come up the insidewalls.

Can the valve or joint behind the wall leak only when water is flowing? :confused:

It definitely sounds like a small leak in the faucet itself. Keep looking around it.

Last night I recheck several things. First I pulled up the sink stopper and filled the sink from the hot water. Think I can still hear the faint drip sound. Turned off the water and listened. Faint drip. Got under the sink and viewed with a flashlight and felt around with my bare hands - no wetness. Felt from both sides of the facet and down both tubes to the valve - dry. Under the sink I heard one or to more drops - then stop. Got up and drained sink and went under to listen. Sounded like the normal discharge - no drip sound. Turned on the hot water again and got under and could hear the drip. Turned the water off (cussed alittle) went back under the sink heard a few more drips and then it stopped. I tried the same procedure on the other sink and what do you know, with the hot water on, the same drip only fainter. Turned the water off a few more drips then stop. I have looked on the ceiling below and don't see any water damage. Does't feel soft anywhere. I am still confussed.

Does this mean ripping out the ceiling? Or something not so pleasant like that? :eek: Could I be hearing a echo through the air vent?

It's very common, We have a similar issue, I don't think it's any leakage .
There are many explanations, following is one of them:

If the sound occurs only when you using warm or hot water that is OK. I bought a brand new house and have the same problem. I asked my builder to check. My builder told me the water dropping sound is not pipe leakage. It is the sound of plastic pipe expending when the hot water running through.Then I try only use cold water or just flush the toilet upstairs then no water dropping sound at all.