They will whistle and howl when things aren't right (struggling to run at low rpm's, especially 1 and 2 barrel carbs). Keep in mind, these older carbs, unless specially noted, probably are not being built with ethanol in mind. I see AFB and Thermoquad rebuild kits that specifically mention alcohol-tolerant accel pumps, the rest of those carbs already fine with alcohol. Maybe the very latest remans might have improvements, but certainly not one made even a couple years ago. There could be something else going on, but just judging by how frequently I have to change the rubber part of my fuel line, I'd bet the carb is tired. Unless it has a metal fuel float, I'd suspect it as being currupted and causing the engine to run too rich from flooding.
I think G1D is right on the mark and there's an issue with the Acc pump. If you're able to exchange the carb again (if it's still under warranty) because it sounds like you bought an aftermarket Holly, I'd inquire as to the diaphram and if the acc pump is ethanol tolerant.So you think it is a carb problem and not a vacuum line?