I have a Perkins 4.108 which is about 35 years old with about 1500 hours. Yesterday, we ended up motoring the last couple hours of a 5 day cruise and I shut down the engine. When I went below I heard a strange noise. At first I thought it might be my refrigerator or my battery charger. I shut both down and still heard the noise, so I removed the cover and it sounded like boiling water coming from the engine. My raw water pump was working and the engine temperature appeared to be normal both by the gauge (about 160) as well as by touch. Does anyone have any ideas of what might be going on?