When toilet flushes makes an awful fog horn noise

If you went from a gravity system onto a Combi then there is a chance that the toilet was being filled with tanked low pressure water and when you swap over to a Combi all cold pipes in the house will now be mains fed, a restrictor is needed to be fitted into the toilet fill valve to stop the noise or you can buy a decent fill valve for under £20 and just replace it

You are now operating via mains cold as you have a combination boiler. The fill valve on your cistern was gravity fed ( water storage tank in the loft) it is now mains fed, therefor a high pressure fill valve needs to be fitted to your cistern. This is a common issue. Good luck.

is all your cold water off of the main now?,if your cold water used to be from a tank your cistern ball valve will have a low pressure orifice,this is why it is making a noise,change ball valve to high pressure type.

If you have just had your boiler changed, try opening the stop cock fully open,this should do it.