In article ,
John Rumm writes:
One reason for writing it up (other than "venting" on a rather less than
entertaining weekend) - prior to then I had never really given the
possibility that one pipe might saw its way through another much thought.

Very common with refrigeration systems.
Vibration from the compressor is the source of the movement.

Doesn't need to be pipe-to-pipe - pipe rubbing against anything hard
can do it, even a bit of loose grit over time.

--
Andrew Gabriel
[email address is not usable -- followup in the newsgroup]