I have a NOISE WHEN I FLUSH THE LOO, that is a loud whooaahhoooh noise. Not a bang or crash at all and not a hiss, sort of between the two. It goes once the cistern fills up again, but the noise is not in the toilet, its in the pipes

Any ideas?

Open the most distant faucet and let it run for a couple of minutes. That should bleed all air out of your lines.