Can anyone help to problem solve this noise from hot water system?

[Jonlovelock]

Hot water system making a very high pitched squeal. Very loud when using the bath upstairs, not as loud in the downstairs kitchen but water pressure has dropped. My step dad (not a plumber) has changed the combination valve but no luck. Any suggestions? Thanks! Attached are videos of the noise, excuse the mess!

 

[SimonG]

Hard to tell from pictures but it looks like an unvented cylinder.

When was the last time it was serviced?

Also need to hold a g3 qualification to do anything with them or their ancillary equipment.