It sounds like the flame arrestor is dirty. It will be a pain to clean but you need to vacuum the lint and dust from it. It is located under the heater in the center. You will have to make some kind of attachment for your vacuum.
Here is a link http://www.broomfieldplumber.com/wat..._cleaning.html
the top water heater picture should look close to what you have. If you are lucky you still have that black plastic screen on your and can just clean that otherwise you need to get inside behind that to vacuum the dust out.