Sounds more like filters to me. That mystery tank could just be one. Trace back and see clean all filters. Water tanks and
lines are most vulnerable to issues as you begin the year. After draining in the winter, filters are usually the culprit...especially if the system is not really clean to begin with.
OH, those filters most likely are at the
pump on the feed side. You could also be having a problem with the filters on your faucets. Amazing what those little buggers collect. When you clean those filters you don't want your wife to see what comes out. Don't drink the water!
Also, to the person worried about your accumulator tank, most boats I have worked on have a water tank and a
pump, no accumulator in between. If you don't find one in any obvious places...stop looking!