If the damper is buzzing i may be that the moex plug is not properly engaged at the plug and there is sone current loss through the low voltage wiring. If it is a field controls damper it is not uncommon for it to buzz when the plug is connected but ajar. Some times the buzzing is coming from a loose sheet metal screw that is vibrating when the damper is opening or if there are not at least three screws in every section of the smoke pipe if it is a single wall pipe, if you have B vent pipe check to be sure that the sections are connected securely. (they should be @ about 60 degrees apart between screws) and that could cause some buzzing when the damper is opening or closing.
As far as leaving it manually open you will loose heat up the chimney but the boiler will still operate with the damper manually open just not as fuel efficiently. Also check the jacket screws and draft hood screws sometimes they can drive you nuts over a loose screw.
It also could be that the motor is on the way out and you would have to replace it.
Here is a link to the PDF file for that model page 48 will give you some information about the damper.
The boiler will not fire if the damper doesn't open (if it does fire with the damper closed shut it down and call a professional) if you want to check for loose screws it may help you out some. But you may leave it manually open all the time in the short term if it does fail but don't leave it open too long or your gas bill will start to pick your pocket.
Good luck I hope it was some help and look over the link to the equipment it may be helpfull