Ideas for diagnosing random terrible noise?

[Rollo Grande]

I realize this is pretty vague, but maybe you've had a similar problem and have some ideas.....thanks in advance for any leads!

I have a new-to-me 1970 C20 350 with the the 4-speed granny trans. G80 rear axle. I don't use the granny so 2nd is first, 3rd is second, etc.

There is an occasional metallic bang/jerk that happens upon acceleration. I am only driving around town at pretty low speeds, 20-40 max, so it's occurring in 2nd gear, sometimes in 3rd, and not in 4th that I can recall.

Felt to me like it was in the driveline, I worried it was the diff going bad, or something in the transmission. Yikes $$$$.

But today had a local crusty old transmission guy (the best kind!) test-drive the truck and when we felt it go BANG he said, "I don't know what that is, but rear ends are pretty consistent when they go bad, and your trans sounds normal for an old truck. I'm pretty sure it's not in your driveline, sounds like it's coming from up front."

So I guess that's good, or at least less-expensive news, but not sure where to look.

Some wild guesses: Motor mount? Frozen wheel bearing? Seized brake part? (The brakes only recently were restored to power assist but they seem to work fine, and the noise was there even when driving with Flintstones brakes.)

What would you investigate next?