Pretty sure the noise is not valvetrain noise. It's more of a rattling clutch type sound, which is always there, but gets very loud when hot.
my 2000 M750 does the exact same thing (read the whole thing before panicking):
didnt notice it when I bought it because I had never gotten it fully warmed up. I did an oil change before I even registered it, and while the oil that came out was black as hell, it was perfectly clean, the pickup tube was clean too. I bought a filter at Napa. over the course of the next 300 miles I started noticing the sound and it got louder and louder. to me it sounded like rod knock, so I freaked. drained the oil and sure enough it came out looking like heavy metalflake car paint, which confirmed my assumption of rod knock.
so I tore the entire bike down, pulled the motor completely apart and did a full overhaul. sure enough the rod bearings each had massive scores in them and some flaking, as if the napa filter had debris inside it. BUT; I plastiguaged the rod bearings and while they were out of spec, it wasnt by much, definitely not enough to make an actual rod knock noise.
So I get the motor almost completely reassembled, last thing I install is the oil pump. As I'm putting it in place, when the gear butts up to the crank gear it makes a knock sound that sounded EXACTLY like the sound I heard that I thought was rod knock; a metallic clanking/chattering noise, similar to an open dry clutch sound. so I get the oil pump torqued and I stick my finger behind it and start playing with the oil pump gear, and sure enough the gear mesh is a hair loose and I can rock the gear in its rotational direction a small amount, and when I do this it makes that very distinct clanking/chattering.
got the bike back together and have driven it a bit since the overhaul, and sure enough the sound is still there but the oil is staying perfectly clean and oil pressure is staying quite high.
so its a good thing I freaked when I heard that noise, because even though it was unrelated, it did cause me to drain the oil and rip the motor apart before anything else got damaged. the only damage was the rod bearings, but the sound was the oil pump gear chattering with the crank gear. there is zero play in the oil pump parts, and no adjustment for the gear mesh, so I assume this is just normal on some bikes