Only thing I can think of is your cam tensioner is not functioning right. It is hydraulically dampening the cam chain with motor oil. As oil gets hotter it gets thinner. The fact the fan is on while doing trail work would not worry me. Did the temp go up??? or was it stable. I have done trail work with mine in the desert at 40 deg. C. with the fan on, and the temp did not move at all. While I have a slight knocking noise, that somewhat diminishes with a pull on the clutch, I have never seen it increase when the motor gets hot. I think the 10-40 is good enough, and would look at perhaps try a new tensioner. There have been a few on here that have theirs replaced due to noise at start up ( which mine does) and it went away with e new CT.
(PS) clutch noise (rattle) is normal, as the plates are loosely rattling inside the basket.