After installing my bath at the weekend, I now get a fairly loud noise when
using the cold tap. The tap is a mixer, the hot is from a cylinder and the
cold from the main. The cold is 15mm up to just before the tap where it
expands to 22mm to connect up.

I don't get the noise when the cold is on full whack, only when it is on
slightly - as I turn the tap on more, the noise decreases until it stops at
about 50% of the way on.

I know that due to the way the hot and cold water come through the bathroom
wall, they are pretty much touching at one point - could this be the
problem? Or is it more likely something to do with air in the pipework?
Don't know if this makes any difference, but at the moment, the branches for
the basin and toilet cistern are not connected to anything.

All help much appreciated.
Jon