noisy water inlet

Recently moved into a small semi which had had a new boiler fitted in June 2014. At random times, sometimes every 20 minutes then nothing for hours and including during the night, the sound of water coming in and then the clunk when the valve closed is horrendous, so loud it over-rides the television. In the dead of night, it is extremely disturbing. I am thinking it must be something to do with the combi boiler as I think its only started since I put the central heating on. The installers are not interested as they say it isn't consistent with the operation of this type of boiler. There are no other appliances like softener on the system, just a washing machine. The mains in and the boiler and the cistern are all on the same outside wall. Don't think the cistern is the culprit as there is no evidence of the overflow dripping or constant water running.
Any ideas gratefully received? Thanks.

Probably a faulty toilet fill valve.

OK but would there not be evidence of the existing water in the cistern going somewhere?

Not if it is only letting by a little, you probably would have flushed the toilet before if would overflow.
Try isolating the supply if you can and see if it stops.

Thanks for the advice, I've just had a bit of a breakthrough. I was looking inside the cistern and coincidentally the noise happened. What I saw was the level of the water go up in the cistern then the clunk of the valve! Your advice seems to be right and I can now get it changed. Many thanks is!