I replaced the outside tap the other day and since then I get an "drone"
noise occuring intermitently when I turn the taps on. It seems to be coming
from the pipework under the sink - when I hold the mixer head at the kitchen
sink I can feel a vibration and the noise decreases. It only does it for
about 5-10 seconds and then goes away again.

Anybody got a clue about what it could be?

Thanks!