my blower to gas water heater keeps kicking on

I love our AO water heater and have never had any problems with hot water since putting this one in on 2002. Untill recently when My exhaust to gas water heater keeps kicking on and off for seconds at a time..usually there is no call for hot water as no one is using any. It runs for a few seconds then shuts off several times in a minute.

As I am 53 year old woman and not familiar with the mechanics of heater..I'm sure it is a easy fix. What can I do to fix this? Not sure where the filter is located and if maybe it needs cleaned? What can I do?