Raypak 11KW spa-pak

I am getting a buzzing noise from my heater, and almost an ammonia smell. The light switch seems to go on and off?  I am wondering if the flow switch is going bad, or I am not pulling enough current. Both 220v pump and heater are on the same 50 Amp circuit.

It is probably the contactor that is humming. This is normally due to undersized wire or breaker. The 11 kw requires a 60 amp breaker and 6 gauge wire.