Foghorn sound from airing cupboard

For several months now, after we flush toilets, or after we turn off either hot or cold mixertaps, there is a deep noise foghorn sound coming from immersion tank area in the landing. The noise lasts for a second or two. A plumber came a fitted a new equilibrium ball valve to tank in attic, but this made no difference to noise stopping. Any ideas? Could it be trapped air in system? Broken pressure valve ? Vibrating pipes? Anything else? Advice welcome!