On Wednesday, 7 September 2016 11:52:59 UTC+1, Dan S. MacAbre wrote:
While tying up the plastic ballcock arm in the loft, I rather stupidly
snapped it. I went out and got a brass one, which seems to be a much
stronger replacement, so no big deal. However, the new one is a lot
noisier than the old one. Again, that wouldn't be a big deal, but when
the tank is close to its full level, and the valve is near to closing,
the noise starts to fluctuate. I imagine that it has created some kind
of resonance in the water, and that adjusting the mains water cock will
get rid of it (I only open it about a single turn). But I have a
question about the new brass valve. The water comes out of a hole
drilled into the bottom of the valve body. Okay, I know it's got to
come out of somewhere, but since it just dribbles and splashes all over
the place, I'm wondering if perhaps there shouldn't at least be some
kind of nozzle to attempt to direct it (like the old plastic one had)?
Am I supposed to buy something else to finish it off? I'm thinking that
this might be contributing to the noise.

Yes first thing to try is turning the isolating valve down as far as is practical.
Have a look at the "hole in the bottom" of the valve.
If it has a female thread you can screw a short bit of plastic pipe into it which may help matters.
This bit of pipe used to be supplied with the valve in days of yore

You could try bending slightly the brass arm the ball valve is on.

This sometimes changes/stops/reduces the wave running back and forth across the tank and might stop it.

Or fit a cylindrical float in place of the ball.

Or hang a small weight on the ball arm near the ball.
If this corrects the problem, fix it on permanently.