Rattle= weak valve springs and/or incorrect valve seat pressures, to agressive ramp on cam dropping the valve to fast making it bounce or lifters not set correctly OR if doing a no no and running an oil restrictor in the lifter boss with hyd lifters its orfice is to small letting hyd lifter slam against plunger cup at higher rpms and lastly, at higher rpms the if hyd lifter and its an edge orfice type to lube the lobe and the hole is to big draining the lifter to fast, or just a worn out lifter.

With cheap mods you will get well above the 5500 mark on mostly stock mtr.
Aluminum roller rockers, mechanical lifters, new valve springs, aluminum or step up to titanium spring retainers to save a bit more weight and ditch those chincy ass valve rotators on the exhaust valve, install 7/16" rocker studs. Just doing that will allow the top end to hit 6500(bottom end is another story), anything over that and you need to look at tulip and dished valves, 3 spring valve springs and a stud girdle

Don't forget, the valve rotators on the exh valve are heavy, inertia and harmonics suck. There was a hypothesis or pratical theory floating around many years ago that said that at certain harmonics(rpm) the hyd lifter plunger would operate against the weight of the valvetrain without opening the valve initially then on the subsequent stroke the lifter would be empty making the valve slam open to aggressive and opening to far(lifter now not touching pushrod, rocker off the valve stem etc)
I never saw or recall any specs or notes on this but it made sense. Never use the rotators OR heavy spring retainers if spinning the mtr high.