Your hybrid water heater will produce different normal sounds depending on the different modes you set it. You can set your heater to a standard electric mode where no sound is produced when you switch it on. In this mode, the heater utilizes just the lower and upper parts of the heating system.
This happens to be the fastest mode for heating water though it turns out to be the least efficient. The use of the lower and upper parts for heating eliminates the functioning of heat pumps and this is the reason as to why no noise is produced.
A little noise of a normally functioning hybrid heater is normal and this is attributed to some noise from the central heating system or the inbuilt cooling system.
This sometimes becomes abnormal when knocking sounds become frequent and it is always good to have the heater checked by an authorized service provider.
The level of the sound produced by your heater should be constant and should not increase from time to time. An increase in these sounds is an indication that the filters are getting clogged with dirt making it harder for them to rotate.
A normally functioning heater will not produce any kind of rumbling sound at all. Rumbling sounds are an indication of mineral deposits on the heating elements and services of a plumber should be sought to resolve them.
No popping sounds should be heard and it is associated with a buildup of pressure. The relief valve of the system is found to be closed in those cases where popping sounds are heard.
Beeping sounds are alerts that an error has been detected in the system. Any time this sound is heard, it is good to check on the error message on displayed to find out what might be wrong and seeking the appropriate service needed.