Just like electricity, before you try to solve any problems with your water heater, you should always turn it off first. You should also wait until the temperature of the water tank drops so you don’t burn yourself. From there, try to tighten the connections on the heater, then examine the heater for signs of corrosion. If you see moisture on or near your water tank, you may have a leak. If you turn the water heater back on and you still hear whistling, then it’s time to give a professional a call.