Noisy Halstead Hero 40?

I have had a noise for approximately a year on my heating system run by a 3 year old Halstead Hero 40. My plumber has changed thte thermostatic TVR valves to non thermostatic, had a new pump (sound similar to bearings going on pump) and now he has replaced the fan on the boiler, but noise still comes and goes on for a while then stops for a while makes no difference if we have water only or heating or heating & water on. Any ideas pleas he does not seem to have a clue and is costing us a fortune!