[egstasea]

My 97 had a similar problem. I hate to mention this but my holding tank was leaking.  After a great deal of investigating and cleaning it was determined to be holding tank. Purchased boat in 6/21..  The problem was the vent for the holding tank was plugged.  Pumping out the tank collapsed the tank slightly which put a crease in the aluminum. During winter had a new tank installed under the aft bed.  Able to get a 41 gal. tank to fit.  Pulling the old tank would destroy the aft stateroom..  Old tank was sealed and with a good deal of $$$ the problem is solved.  Hope your problem is simpler. Jack

As a side note to the collapsed tank issue, I installed a TEE in the hose before the vent filter, and lead a hose from the TEE to the engine room ladder ending in a quarter turn valve.  Every time I pump out, I open that quarter turn valve so there is an other way for air to get into the holding tank.  The idea is to prevent the collapsed tank Jfhass describes.