I have a 00 Luhrs with twin Crusader 8.2L engines with less than 200 hours with a single 400 gallon fuel tank. I started hearing a noise from the cockpit last week while on a fishing trip. The noise occurs constantly when then the starboard engine is running and for about 1 second when the starboard engine is first switched from off to run (i.e. electricity if first supplied). The port engine sounds perfectly fine. My initial thought was that it was a plastic on plastic rubbing noise, but someone else (whom is more knowledgeable than I) said it sounded more like a suction / vacuum sound. By having someone switch the engine from off to run I can get into the engine room and track the noise too directly beneath the fuel tank, nowhere near the engines. Unfortunately a 400 gallon fuel tank is not just lifted out of the way to get a better view. By closing the master fuel valve off you can stop the noise immediately. I talked to yard mechanic and they do not want to mess with fuel lines since I have a full tank of fuel, they want to wait until I get the tank down to 3/4 or less. Unfortunately that is about 100 gallons of fuel I need to burn and I am somewhat afraid of taking the boat about for 3+ hours to burn the fuel with a potential engine problem (not to mention the 200 bucks) if there is something I can least investigate in the mean time. From doing some research I am thinking that it may be an anti-siphon valve, but any thoughts, comments, or suggestions are more than welcomed. 
