Whistling gas hot water heater

we have a gas hot water heater, age unknown (we know for sure it is at least 3 years old). whenever the tank starts to heat up, there is a loud whistling noise. the whistling stops when the water has been heated. it seems to be in direct relation to the burner. the heater is a StaKleen 805. any suggestions?

2 things, air going into the air intake portion of the WH is making the noise, it might also be the gas line / valve.

Did this noise just start or is it something you just noticed?