Water pressure and boiler

[Brevitt]

Hi, woke this morning to no hot water. While hot water tap was on the water pressure gauge showed very low. Turned the tap off and gauge returned to normal. Checked the pressure dial on the boiler - it was in the green.
Turned on hot water tap, water pressure dial showed normal but boiler made a slow deep ‘thrum thrum’ noise.
Is there anything obvious other than getting the plumber in? TIA

 

[Brevitt]

When hot OR cold tap is turned on, the water pressure dial plummets!

 

[CountryFan]

What make and model of boiler? Can you post a photo and explain which dial you mean? Or is it the pressure gauge?

 

[Brevitt]

Not to worry, plumber came and is ordering parts! Thanks anyway.