There is a continuous noise from the DHW which disappears as soon as a hot tap is opened. After 20 minutes of so the noise re-appears. The central heating system was totally renewed 3 years ago. The boiler is located in the attic and the noise is becoming unbearable during the night. A local heating engineer who cleaned the "muck" out of the DHW filter. Within days the noise has returned and the engineer is off to sunnier climates for a couple of weeks. Checking the manual, the DHW may require a pressure reducer, check valve and expansion vessel. I don't have this. Would this solve my problems?