Drive Train Noise

[Joe/Ct]

I'm stumped. Our '28 roadster has developed a noise when we are driving.
It does not occur in first or second gear (at least we can't hear it over the engine noise), but only third gear. It's like a continuous rumbling noise, that only appears when under load; when I take my foot off the accelerator or push in the clutch, it goes away. The oil level of both the transmission and differential have been check and both are full. Any ideas?

Also, is there a problem to continue to drive it this way?