[jeffcoslacker]

I'm thinking you are hearing some kind of vent or pressure release valve in the tank that is not working quietly. Ball check type valves can make buzzing, whistling, wheezing and a variety of other sounds, depending on the design.

About your change over, you'd have to do some research with the conversion supplier to find out how involved the conversion to flex fuel is, and reverse engineer it from there. But the problem is, you may run into emissions non-compliance problems, like all those who converted their diesel Buicks and such to run gas. The Fed says it's gotta have the emissions controls it came with from the manufacturer. If the flex conversion includes any "unique" sensors or controls, it may not pass a legit visual inspection. Suck to go to all the trouble and not be able to license it. I'd find out first.