Sorry to hear about your prb, David. Have you tried my very non-scientific way to see if the noise cuts off if you brake hard? ***If you want to test this, please do it on an empty road w/ no one behind/around you.*** When I brake hard (when I had the noise), the noise would cut off and go back on, almost as if the sloshing fuel had something to do w/ muffling the sound. Mine had the same noise as yours but now it's gone, even though your mileage is higher...who knows, maybe it'll disappear like mine.
...but of course, you should really get a good solution instead of the dealer ignoring what is an obvious prb here.