Wayne:

Thanks for the info. As for condensation, that is possible, although I
am in Northern California, so cold is a relative concept. We've had a
real cold snap lately (hi 20's-low 30's), but we don't usually get
below the low 40's/ high 30's, and this problem started before the cold
snap (although not before the generally cooler winter weather).

I think air is the cause of the buzzing; it often happens after I've
opened one of the filter canisters, and when that happens, it will stop
if I open the output valve and let it run for a minute. On the other
hand, when I recently changed the filters, I let it generate a couple
of gallons of output to flush the new elements. Shouldn't that have
bled any air out of the system? The buzzing seems random; it will be
fine for hours and then all of a sudden buzz it will buzz for several
minutes- sometimes as many as 10-15 min. Perhaps it is related to when
the automatic float lets some water in to the refugium. Is there a
specific procedure for checking/bleeding air? The only other thing I
can think of is if there is a problem with the membranes; they are only
7 months old, and tap water is good quality, but its treated with
cloramines, which I understand can shorten membrane life.

- Dave