Got a boiler that has me scratching my head. It gurgles. When a zone calls for heat it fires, runs to supply heat, comes up to temp once demand is met it shuts down. Then it starts to gurgle, not every time but maybe every 4 or 5th cycle. All air vents work, thoroughly cleaned and brushed, drained and filled, pressure and temps are good. 20 year old Burnham in good shape. These folks want a few more years from this thing so replacement is a bad suggestion. Told them a new mpo would be a good way to go.