G'day all,

   Thanks again for the many helpful responses about the C & D notes
playing together.  I have posted here two of the excellent responses in
case they don't appear elsewhere ( I hope that's OK) and maybe some of
the others might surface also.  The prize for the simplest solution
goes to Craig Brougher's suggestions below and guess what --- when I
pumped out the tracker bar, the problem stopped!  I confess that I
hadn't done it for a while and there was quite a bit of dust.   It
returned again less severely after playing a few rolls and only happend
at low suction.  Pumping out again has stopped it for the time being.
It seems to me that I probably have a poor seal under some pouches and
partially blocked bleeds agravated it.  I might still have to open it
up to get rid of the problem altogether.