There has developed, in the last week, a "fog horn" sound when we turn on the hot or cold taps in the bathroom or kitchen.
It does not occur when we use the washer or dishwasher. Could anyone give some advise on what could be the cause of this?

First I would turn on a faucet until you hear the noise, I would then leave the water on and go to the commodes and lift the lid off the tank and see if you see any kind of vibration in the water in the tank. I would then lift the float just a little to see if that made any difference, I would do the same thing to the second commode if you have one. If this changes the noise you will have to change the valve in the commode making the noise or perhaps changing the water level might do it.
This may not be the problem but in cases I have found this to be true. If it is not then you will just have to search for the problem, you can start by turning off all of the supply valves that serve the faucets and the commodes and also the washing machine. Then open the supply stops one at a time, then turn on the faucet, say you turn on the hot water supply stop than open the hot water of the faucet. If you do not hear the noise then shut the supply stop again and go to the next one and repeat the above till you have found one that makes noise. I am not sure this will help you find the problem but it is about the only thing I can do from this far away.