I just moved in to a 9-year old house. When I am taking shower or using the faucet in 2nd floor bathrooms, there appear to be a slow dripping noise in the drain pipe during draining. During the water draining (either cold or hot water), the noise sounds like da... da... da. But appear to be at a long interval occurs at one location. I try just running cold water, the same sounds occur. The only time that the sounds disappear is when I drain the water very slowly. I am not sure if it is a leak or just wood contraction/expansion. I check the first floor ceiling visually but didn't found any wet spot. Should I be worry? Is there a way to find out if there is a water leak without an opening up the ceiling

(Note: When the home inspection was done during Sept, either the inspector or I did not hear these noise. However, maybe we missed something.)

I am not sure if it is a leak or just wood contraction/expansion.
BINGO!! I think you've located the culprit. Knocking, popping, creaking and crackling pipes are a common complaint, especially in colder weather. I can tell you what's happening and take the mystery out of it but you're not going to like the repair. When a draw is made on a hot water line the line expands against a pipe strap next to a stud and when you drain hot water from your tub or shower it goes into a chase that is a little cooler then room temperature. The heat expands the pipe causing it to rub against the stud to which it is pipe strapped. This is the sound you hear. As it cools it contracts and the noise is heard again. To repair it you must tear open the walls and locate the pipe strap that's causing the problem and shim it tight. Most people when they learn what causes it just elect to live with it. Regards and thank you for rating my answer. Tom