Grinding at higher RPMs 1979 300TD

[BenCohen-300TD]

Well, I've searched and searched, and there are lots of posts about grinding noises, but nothing that describes my situation:

1979 300TD non-tubo, 139,000mi. New (rebuilt) transmission shifts great.

When accelerating, at higher rpms, I SOMETIMES get a grinding noise that sounds, to me, like a bad bearing. The sound is correlated with the engine RPM, so I also hear it at highways speeds, but not all the time. I haven't heard it in a while, but my GF reportedly heard it driving on the highway earlier this week.

If it's a bad bearing, where could it be? Since it's correlated with engine speed and not wheel speed, it's sounding scary to me.

Any thoughts? Thanks everyone,

B