howling noise from vaillant c/h boiler

hi there

my 7 year old vaillant c/h combination boiler works fine except that when i turn a hot tap on and off whilst the heating is on, a loud howling noise starts from the boiler. this only happens when the central heating is on, and once it has started it can only be silenced by turning off the unit completely. the vaillant engineer has replaced an internal pipe (no difference) and suggests the heat exchanger may need replacement. (very expensive) but can't explain why. i think he hasn't a clue.
any ideas or suggestions greatly appreciated.

thanks

I would without seeing it say its the H/e has furred/scummed up an needs replacing ,

When you engage a Vaillant Engineer through the Company then you should expect a proper diagnosis of the fault.

My advice assuming you actually did pay for the visit is that you should complain that he was unable to diagnose the fault and they should sent another ( better ) engineer at no further cost!

It can be difficult for some people to diagnose faults!

Tony