Hot water heater bang

Sometimes it does ok. Sometimes it goes BANG when it lights. I just turned it on with no problem . it lights fine at first then when it tries to relight it happens,,sometime and sometime it lights just fine. Thay stuck this HW tank under the kitchen cabinet/sink area i have no clue how to get to the back side of the tank to take off water lines if i have to remove and replace it or to install the thermostat for the hot rod electric heating thing. No access pannel at all.