Engine rattle - 3500RPM or so

[JJorgensen52]

Guys, I'm pretty stuck on this one, so hoping somebody might have some insight:

1972 C3500 GMC, 350 with 4 speed, PS. No A/C or A.I.R. stuff. Does have the standard stove pipe riser on the driver's manifold

I get an irregular rattle coming from the motor above ~3500RPM ... best guess, no tach. It occurs under any load condition or throttle position, appears to be entirely RPM dependent - no noise at low RPMs, noise at high RPMs. Doesn't vary with RPM once it starts, at least that I can detect.

It sounds not entirely regular, like it's not a rotating part. I'm thinking potentially the heat riser pipe / manifold shield, or something to do with the exhaust? I can't seem to find anything, though.

The motor runs as smooth as glass, has 50k miles original on it, never been opened. Hardly burns any oil, everything looks clean inside so far as I can tell.

It will make it more or less continuously driving on the highway - I start hearing it at 65 and it doesn't really change with RPM above that.

Any ideas? Thanks in advance.