That part about chattering worse than the old 2 flute HSS is what gets me, because I used to argue the opposite. I always said more flutes means smoother cuts, no question, and told guys to ditch the 2 flute stuff. Then I ran a 4 flute in 6061 on our old machine and got the worst noise I ever heard. What changed my mind was learning that 4 flute tools have less room for the chips, so the aluminum gums up and starts rubbing instead of cutting. Once chips pack in there, the tool pushes back and forth, and that is your chatter. 6061 is gummy enough that a 2 or 3 flute with good chip clearance usually runs way better, even if the tool is carbide. Shallow depth can make it worse too, but I would check your holder and your flute count before blaming the speed and feed.