How can vibration/noises in Glow.worm 18hxi be fixed? ?

Three-year old Glow.worm 18hxi continues to make vibrating noises. Installed new pump and automatic air venting valve etc on professional advice with some lessening of problem. Vibration/noises mostly heard in evenings, seem louder when general temperature drops in winter evenings and occur just as boiler cuts out when water has reached the set temperature. Advice, suggestions or useful information gratefully received!