My water heater model number PR6 40 NBRS sometimes wistles and I can hear the water inside boiling even tho water coming from taps around house in not much hotter than usual. This has only happened 2-3 times in the past 6 months. I usually turn on kitchen faucet and drain a little water from water heater drain valve. Someone wrote on it that the heater was installed in March 2000. Is this problem dangerous and should I have it checked by a professional?