On 07/09/2016 16:53, Dan S. MacAbre wrote:

Yes, I don't think it would take much to disturb the equilibrium of the
whole thing. I expect it can only be oscillating like it does because
conditions are just right :-)

My MiL had one like that - the feed into the loft tank went through a 90
degree bend a couple of feet from the tank, then went across the loft
for 10ft, all unsupported. I tied the bend to a rafter with a bit of
string, and that damped the vibration enough to shut it up.

Andy