oil burner rumbling on start and stop

the furnace guy was just here a few days ago doing maintenance. He adjusted the flame height by eye, no instrument. Today I suddenly noticed a loud rumbling both when the burner starts and stops. I experimented by rotating open the peep hole door a little to give it more air, and the rumbling stopped.
I believe I do see how to adjust the air intake, which I want to do until the furnace guy comes back. It looks like a collar that is rotated around the same axis as the motor shaft. The collar is to the left of the whole burner assembly.
1) Does the boiler rumble because it has too much air or not enough air?
2) Would it be bad to leave that peephole door ajar for a few days, instead of adjusting the air intake? It would let some cold air come down inside, but is there any other problem?
3) Why would this noise have suddenly developed? Is it that something went wrong inside the works?
Thanks.
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Perhaps what you are hearing is the resonance vibration inside the chamber when the flame starts and stops. It could be it was not there before because maybe your air intake was set wrong. Most techs adjust the flame by eye. They look for a white flame when adjusting the burner. Chances are the flame is now set right and the rumbling is normal. I have a rumble also when mine starts. It just indicates there is a strong flame burning.
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thanks for the reply, Mike - but this is a really loud rumble, enough that I think it'd harm the furnace or pipes connections over time because of the vibration
plus, it was okay for the first 3 days after the furnace guy had been here. The rumble developed on the 4th day.
RJ_32 wrote:

Mikepier wrote:

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Get him back to make proper adjustments. Can't be good in the long run, but it is usually a problem with the air/fuel mix. Something was probably loose and moved.
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