Please can someone help me!

My problem is this:
For the last six months I have had a loud whistling/hum noise from the engine that rises and falls with the pitch of the engine.

It is more audible when the engine is cold but is still there when everything is warmed up.

I got the dump valve changed but this didn’t solve the problem.

No one seems to be able to track down the problem.

Any ideas because it is literally driving me mental!

Thanks