Last night around 11pm there was a brief but loud foghorn noise I wasn't sure the origins of, but then this morning around 2am when someone went to the toilet and flushed the foghorn noise came back!

The thing is: it does not stop at all unless we turn on the shower pretty high (all cold water). We managed to actually turn off the water once this morning and have it silent but then someone else went to use the bathroom and... well, needless to say, the foghorn noise is back unless we turn on the shower.

And it actually does not stop. It's past 7pm now (have sent people out for spare parts just in case this is the same as all the other asks and it's a toilet problem), and what worries me is that I've turned off the water to the toilet, but the noise continues.

Could it be something else that can't be fixed DIY?

Lift the lid and check the tank level. It should be one half inch under the white overflow tube after the tank is filled. Do you have a float ball pipe or a float can for a ballpark? Let me know. Back to you, Tom