Why does the geyser hiss after turning off the hot water?

Hello. In my opinion, the following reasons may be most likely:

1. Perhaps the gas valve sticks, so it does not turn off immediately after turning off the water, which is accompanied by a characteristic sound;
2. The reason may be the scum that formed in the tubes, and when you cool you hear a hissing sound;
3. There may be problems with the pilot valve;
4. The reason is too high gas or water pressure.

You can check and confirm the last reason (or refute) yourself. If this is not the case, then you will have to call a specialist.