carver water heater, intermitent problem?

Ihave a carver cascade 2 water heater which is very good but every so ofter when the heater is switched on and working there is loud popping sound when the burner ignites, as if the gas burner is realeasing gas but does`nt ignite for a few seconds which is probably causing the popping sound. It does`nt happen all the time and the water heater is so good but every now and then this problem occures. After the popping sound and the burner ignites the water heater works as normal.
Thanks guys.
Kelvin.