It is an air lock and the easy way to fix it is to go to the taps furthest from the boiler and put a pipe between them (garden hose type thing). Turn the hot tap on 1/4 – 1/2 and then the cold on slightly more than the hot (pushing cold water up the hot pipe). Run for a few seconds then turn them off and go to the boiler (if a combi) and release any excess pressure built up (red dial on pipes underneath that says 3 bar on it.
Repeat this for all other taps in house working farthist from boiler back to closest.
If this doesn’t work, there could be a leak in the heater matrix or expansion unit and will need an engineer.