Boiler or Megaflo making whining sound...pulsing in waves

[Charlie BH]

Hi There

This is my second post - I wonder if / hope that anyone can/might help. We have a Glowworm Boiler or Megaflo making whining sound at home...pulsing in waves (see YouTube video here:

Is an unvented cylinder megaflo - maybe I should replenish the air gap and see if that helps?

Thanks

Charlie BH