1. The noise happens after hot water is used, such as after having a shower. A knocking sounds like a finger knocking at the wood.
2. The knocking sound is in predictable frequency, from every 20 seconds to 90 seconds, it seems the colder the weather, the higher frequency and louder noise. In hot summer, there is no noise.
3. In next several hours, the knocking sound will reduce in frequency and volume gradually, and eventually disappear. In summer, the sound disappears faster.
My guess is, the hot water pipe make noise after it is hot, and reduce its magnitude and frequence as it is cooling down. However, I do not understand:
Why the hot water pipe make noise? I cannot image the hot water pipe will make several hours of noise without any external force, and why is it in such predictable frequency?
Any suggestion to fix it? It is very difficult to find out where the noise from without taking house apart? Any technique I can use to isolate the knocking location?

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. Good luck and thank you for rating my answer. Tom