Cause of banging pipes in bathroom?

Install a water hammer arrestor.

The pipe is not fix priperly and when is under the presure si move

The banging or knocking noise you're experiencing when you turn on the hot water tap is commonly referred to as "water hammer." This phenomenon can occur due to several reasons, but it primarily happens when the flow of water in the pipes is suddenly stopped or changed, causing a surge of pressure that leads to the pipes moving and banging against walls or other pipes. Here are a few potential causes and solutions:

High Water Pressure: Excessive water pressure in your home can cause water hammer. This is more noticeable with hot water due to the expansion of pipes. Reducing water pressure using a pressure reducing valve may help.
Air Chambers: Modern plumbing systems include air chambers or water hammer arrestors to cushion the shock wave created by the sudden stop of water flow. If these air chambers become waterlogged or are absent, water hammer can occur. Draining the system can sometimes reintroduce air into these chambers, but since you've mentioned this hasn't worked, they might be malfunctioning or absent.
Check Valves and Pipe Mounts: Faulty check valves or loose pipe mounts can exacerbate water hammer issues. Ensure all pipes are securely fastened and check for any malfunctioning valves that may need replacement.
Thermal Expansion: When hot water flows through the pipes, they expand and can rub against structural elements, creating noise. This issue might be mitigated by adjusting the path or securing the pipes more firmly to accommodate expansion.
Faulty Taps or Appliances: Sometimes, specific appliances or taps can cause water hammer due to the way they shut off water flow. If the issue is isolated to the hot water tap, consider inspecting or replacing the tap's mechanism.
Solutions can include installing water hammer arrestors near the source of the noise, ensuring the plumbing system is correctly pressurized, securing loose pipes, or consulting with a plumber to inspect and possibly adjust or replace the internal mechanisms of your taps. Given the complexity of plumbing systems and the potential for damage if not correctly addressed, consulting a professional plumber for a thorough diagnosis and remedy is often the best course of action.

Can be air in system or faulty tolite filling valve or a faulty tap can cause noise in pipe system

switch of water supply
turn taps on top of system and flush toilet shower etc
turn taps on bottom of system
turn water back on with all taps open
run until even flow
this called water hammer