If the circulator is making a lot of noise it's probably the coupling between the motor and bearing assembly. The B&G coupling is a two piece affair with 4 little springs that connect the two pieces. They wear out and break. Changing the coupling is as easy as removing the 4 bolts that hold the motor to the bearing assembly and backing off two allen screws that attach each half (one to the motor, one to the bearing shaft. Re-assemble and oil the bearing assembly. If it's not the coupling the bearing assembly itself could need replacing and that's a bit more of a job as the pressure needs to be relieved from the boiler.