I purchased a brand-new 3-story townhome May 2006. Everything looked and sounded fine when we moved in. In November, I started to hear this "tickering and water-dripping" sound from the pipe every time I run the hot water from wash basin in my bathroom (only the basin in mater bathroom has this sound, all other rooms are fine). The plumber came in to check and told me it's the result of pipe expansion and shrieking, and it will go away in spring and summer when there is not much temperature difference in- and outside house. But I am still hearing this sound, even though it's summer now. Can anyone please tell me why this happen and if I leave it as is, will it damage the pipes and cause leaking in the future? Thank you!

It can happen in the summer also. In the winter hot water expanding the pipes is the culprit, in the summer it's cold that causes pipes to contract in a hot chase in the wall.
Let me explain, 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, Tom