I was getting the "water noise behind left side of dash" issue about 2 weeks before the pipe was burned through - they said it was probably air in the systerm and that it should be resolved as part of fitting this new pipe and refilling/bleeding the system.
Interestingly, the Master Tech did say that he thinks they had changed the design of this pipe (from the turbo, downwards to somewhere or other) since I got it so maybe there was a known problem and a design change was made...