Any ideas what this could be? I have a condesing boiler with no hot water tank. When I flush the toilet up or downstairs or use a tap hot or cold tap I get a fog horn type noise coming from the pipes and the shower head drips water.this doesn't happen of the dishwasher or washing machine is used but when the sinks or toilets are used and stops as soon as the toilet finishes filling or I turn the tap off.