Rattling lifter-type noise above 150°

I have what I think to be a bad lifter making noise above 150° operating tempurature. I'm a little puzzled as to why it only starts above that temp. At startup I get no noise until the engine has heated up some, then after that it is intermittent. I have listened to the noise with a mechanics stethescope(not just a piece of hose stuck in my ear), and the noise is most noticable around the bottom of the valve cover, nearest the bolts..not on the top of the valvecover at all, and is quite pronounced nearest the manifold

My theory has always been that when the oil heats up and thins, it isn't enough to hold pressure in the lifter. Is this a possible explanation? I removed my valve cover and checked each lifter by pushing on the pushrod, all lifters resumed pressure normally, I also checked for bent or otherwise damaged springs and pushrods to no avail. I didn't check the torque on the rocker arms because I didn't have a torque wrench at the time.

Is it possible for a cracked exhaust manifold or loose manifold bolts to imitate the sound of a bad lifter?

Is it possible that the rocker arms are not quite to spec and as the engine heats up clearances get a bit bigger causing the rattling?

Is it possible for the noise to be a worn wrist pin or just continuous piston slap?

Thanks for any insight regarding my questions.