I often hear some cracking noise coming from the water pipe (hiding inside the drywall) wheh there water are not is use. The noise usually come after 5-10 minute after a shower is used. BUt not right away

Sounds like the hot water pipe is cooling down after use and stressing against clamps or material used to secure them. If your hot water is plentiful, you might want to turn down your hot water tank a few degrees. The difference in temperature may ease rate of expansion/contraction resulting in less or no more noise. This would be an easy way to test my theory.

I think you should check water pressure -- should be around 50 lbs

Test by putting pressure gauge on outside faucet -- no mess inside

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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