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All the other answers are absolutely right in the diagnosis with out really looking at your unit...
There is one thing you can do your self, is check to see if there is a draft by observing your pilot, stand as far back as you can to where you can clearly see the pilot and as low as you can from the unit and very quickly for ( just a quick moment ) look to see if the pilot is flickering around and kinda downward, if so you have a draft problem make sure you have your excess cover on and is not missing any and installed right, if it is flickering a lot look to find out how the draft is coming in, use your common sense it could save you a lot of money for an easy fix, If there is no draft please stop and call a certified reputable heating and cooling professional, all there prices are based and set on a national estimating guideline, alto some will give a little better deal, and others charge about 15% over the guideline just to cover their self if the job take a little longer,
You may want to go ahead and just have it serviced even if it was a draft problem and you were able to fix it,
Please know you and your family are safe!
In any case it's worth it to protect your self and your home