Foghorn noise on Glenhill Cylinder when drawing hot water.

Possibly a failed tap cartridge or jumping washer.

Without hearing the noise it’s hard to say, I have had issues with in the past with negative pressure on ceramic disk taps, I would check to see if any ceramic disk taps are in the property if so isolate these and try draw off the hot water again until noise stops if noise stops, repair the tap in question and see if that solves your issue unfortunately it can be a process of elimination.

It could be your toilet making the foghorn noise, often this happens just after flushing the loo the noise will come from a faulty inlet valve which sits inside the toilet cistern, but unlikely if you find it's when you draw from the hot supply as toilets should always be fed off a cold supply, but still not unlikely and definatley a possibility. As the others have also said, it could be a faulty monobloc quarter-turn lever kitchen/basin mixer tap. You do have certain taps which are only suitable for low pressure gravity fed systems, so if you've recently converted from a gravity fed system to a high pressure system you might need to change one of your old taps to a high pressure one since you've had this new cylinder installed. There's lots of possible different causes for a foghorn type noise, It's hard to say exactly without carrying out a full investigation into your home.