noisy water heater

[tumsch]

have electric heating and water heater. sure boiler is very old, been vibrating/rattling noise when water timed to heat up at night (economy 7). for a while presumed was rattling pipes after checking a bit more noise seems to be coming from bit on the top that diagrams online say is immersion. heater works fine, just waking baby. what would cause this? and how do i fix?
thx for any help
karen

 

[namsag]

turn it off straight away sounds like its boiling up can lead to vary dangerous situation, where life has been lost.