Furnace starts with a roar

Hey folks,
I have an old oil furnace driving a forced air system. Sometimes when the furnace kicks on it makes a loud thunderous sound that lasts for about a second. I'm pretty sure it's not the air ducts since this happens before the blower kicks on. Any ideas what this could be and could it be something to be concerned about?
Thanks, Nate
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Yup...What is happening is your furnace dosen't light right away. You have to clean the burner igniter. I get the same thing when this happens in my hobby shop in the back yard. An oil fired furnace does not have a pilot light like a gas furnace...it has to relight itself everytime it is about to cycle heat. What is happening is when the furnace starts it's heat cycle, it starts injecting Deisel fuel into its burner but in your case it is not igniting right away so fuel starts to collect...then when it does light it ignites with a big roar because there is lots of residual fuel that has started to burn....after the flame catches up...it settles down...usually after a few seconds... get your furnace cleaned by an oil burner mechanic.... Worse case escenereo is having the furnace fire up and not light for about 15 seconds or so and then there will be LOTS of unburnt fuel in the system...then have it eventually light which will fill your home with black smoke and deisel fumes and possibly rupture your heat exchanger... Yes.. this is something to get looked at ASAP Jim
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