Sounds the same as when my fan went in my combi boiler...defo needs a service likes gasman says - he's the 'man in the know' after all!! ;)
If it helps, I called my plumber out 3 times about it and he didn't fix it (can't expect them to know all the problems in all the boilers I guess). I called the boiler makers Vokera and they came round and (typically) it worked fine so they cleaned the fan instead for the want of something to do. Then it went again so I called the Vokera up and they came out free of charge to fix it.
Good luck