The difference is in the nozzle...
In reply #10, Kirbyklekter asked about the difference in sound between the Heritage II Legend with its new Amodel fan and the Legend II with its (presumably original) Lexan fan. Things have been a little hectic of late but I finally had the opportunity a little while ago to do a side-by-side comparison. It's actually the first time I've had the opportunity to run the Heritage II Legend since I put the new cord on it. Neither machine is appreciably louder than the other but they do sound quite different. The Legend II has a bit of a low growl to it while the Heritage II Legend is a bit higher-pitched but also more melodic, if that makes sense. Of course, then I stuck the hose on the Heritage II Legend and used a Zip Brush to get some dried cat barf off of the sofa and it turned into a real screamer.
Then, as I was taking he hose off to put things back together, I decided to be semi-scientific about the whole thing and swapped the nozzles to account for other variables. Lo and behold, the difference in sound was from the nozzles, not the fans!!! While it's possible the two brush rolls have differing amounts of wear, I think the more likely explanation is that the Legend II nozzle has a moderate amount of crud in it, which served to deaden the resonance, while the Heritage II Legend nozzle is almost completely crud-free, thus maximizing the resonance. To completely eliminate nozzle variations, I made one final test, running each for about three seconds with a bare hose instead of a nozzle. Unlike the previous tests, I had to swap my one hose over from one machine to the other but I could discern no major difference between the two.