I have a 95 yzf600 with 19,000 miles on it. I notice a rattling noise around 5 - 6.5k rpm. Everything below and above that range runs smooth. doesnt rattle when in idle. sounds like its coming from the engine. not sure if its the cct or something else. any ideas?

Check to see if the headers are tourqued down properly. They may have come slightly loose and may be rattling at certain RPMs / vibrations.

Just my $.02

check all fasteners ,exhaust header, clamps, everything, if all is tight, get a mechanics stethoscope, or a long screwdriver and put it up to your ear, put the bike in the rpm range that you hear the noise in. and pin point the noise where its coming from. more then likly its the cct, and or worn timing chain guides. do a oil presher test too. just to make sure its not a oil pump related problem ive seen a few times where the oil pump pumps fine at high rpms but is worn and presher is to low at low rpms and its knocking-rattleing till u get the rpms up and it starts pumping higher presher..... but its more then likly the cam chain tenchener,