The engine and Gearbox share the same oil sump. Unit Construction.
If it's the alternator bolt that holds the impeller onto the alternator, that noise will usually go away at idle if you turn on the high beam.
Or if you reach about 2500 to 3000 rpm's.
There is another bolt in the alternator drive that causes similar problem it bolts the other impeller to the drive mechanism in the engine.
That one is a little more difficult to remedy, but not too bad from accounts I've read here.
Ah right, I hadn't realised it was the same oil sump. Good to know.
I'll try out that alternator rpm diagnosis trick once I got the new intake flanges. It wasn't idling very well at all before and when I took off the carbs to diagnose I completely ruined them I guess, since the rpm went through the roof when I put them back together due to vacuum leaks.