I think you should be pumping the tires to a higher pressure--you're a big dude, and those aren't the lightest bikes. But then, I'm skinny and paranoid about throwing my wheels out of true. You say yours is pretty true.
Maybe you could describe the actual sound a little more--is it high-pitched or low-pitched? Sudden or slow-rising? Groanie or more like a click or whir or...?
Might want to invest in some cone wrenches and take that hub apart yourself.
Does it make the noise off the bike at all? Or only when you're riding?