Just a brief update on this:
Saturday, the problem was particularly bad, and I had to switch the system off a couple of times to get a bit of peace.
Today (Sunday), it started its tricks again. I gradually turned the pump to a different setting.
If you know Grundfos Alpha+ pumps, you know that there are basically six settings:
1) Small system (constant load)
2) Medium-sized system (constant load)
3) Large system (constant load)
4) Normal resistance (variable load) (this is the factory setting)
5) Low resistance (variable load)
6) High resistance (variable load)
Setting 4) was in operation. I turned the pump up nearer to 6), and the problem has gone away, if only for the time being.
Of course, the pump and the sounds in the pipes are now more noticeable.
Does this give you gurus (Cisterns Administrators?
) any further clues as to where the problem lies?
TIA