same type of pads on both bikes ? (should be resin on the bike with the RT56 rotors)
if the bike with the XT / MT800 rotors has metal / sintered pads - this could account for the additional sound (?)
I have old XT (RT81) rotors on one bike and RT56 rotors on another bike - both with resin pads - and the XT / RT81 combo is quieter (and has much better feel / power )
Both wheelsets are used on the same bike, resin pads. I should add that the XT rotors are on a carbon wheelset whereas the RT56 are on alloy wheels. The carbon wheels might just amplify the sound a bit.