my vaillant combi boiler (13yrs. old) has been reg.serviced (BG)since new and has started to make hammering noise on burner start up...at reducing regularity for about 10-30 secs. then it`s quiet! Can only hear this noise upstairs as if on pipework as there is no noise when standing downstairs next to boiler.Had BG out three times to investigate they added Fernox silencer once I have added it myself twice which appeared to quieten things down but now back.BG eng. said age of boiler problem hardwater area,furring up etc.No "kettling" noise from boiler when running/startup.I wonder if it could possibly be a pump problem as the noise disappears after 30secs.or could it be that the boiler is due for renewal???