fluctuating water temp

I have a Valliant Combi boiler..only 3 years old. The CH is ok but when i turn on the shower or any taps the hot water temperature fluctuates and there is a strange kind of rushing sound ( a low whistling sound ) in the system somewhere...pipes. This sound also occurs when the CH comes on. Any ideas?
Thanks

Sounds like the modulation is not working correctly.
Could be the modureg valve.

Noise fault could be Sludged up Gianonni heat ex. Get system cleansed and include boiler in the loop.

Temperature fault could be overheating due to above and most likely is that.

or for other reasons such as incorrectly set up gas valve, which these days on that boiler involves an analyser, and sensors.

Bet now you wished you had got an installer who cleaned the system properly 3 years ago.

Get a magnaclean or boiler buddy fitted after the clean up job.

Hope you don't have to buy a new gianoni they cost the manufacturers £100 so they will probably want £400 out of you. If it comes to that you can get a baxi with the same heat ex in it for about £500 on magic boiler scheme through pts (no connection).

thanks for that guys
sounds bad...lots of money to fix too. I thought these boilers were good. When i had the flat done all the plumbing was done too, all new, all new radiators...the works.
i had no idea it needed fixing again so quickly.

The boiler being Vailiant is very good, potentially the installer was not.

Any recommendations for a good honest plumber in west london?