Sound like a foghorn?

THE issue is the ingoing water pressure i take from your comment the wc is connected to mains you could fit as PRV to the mains or if you know who is the manufacture of the inlet valve you will probably be able to get a flow reducer for the inlet valve

w.c.neil
August 2014

Thanks Rich T just for the record
I have just eaten 4 MCviti Rich tea biscuits, funny that! Will sort it during the week.

bob
August 2014

It really doesn't matter about the make or model, chances are the syphon is on it's way out,they are a generic part bought off the shelf for about a tenner.Have a look at the type,take a picture, if you have a tape measure, measure the height from the bottom of the tank to the top of the syphon (they vary some 9" some 91/2" if I remember right-the lid won't go on with the taller one) any DIY sells them, then follow the instructions on the packet and replace it.Good luck,it is a straight forward replacement.

Rich t
August 2014