Front end of camshaft noise

[LizardsA]

I'm busy rebuilding a '30 Pickup engine, and while it is still on the engine stand, there is a knock as I turn the engine. I have only just installed the crankshaft. The camshaft is in the engine.
1. The timing cover is NOT on the engine yet
2. The timing gear is on really tight
3. With the side timing cover off, the noise appears to be right in the front end of the camshaft, in the block.

What could be causing this noise?

Mike
Johannesburg, South Africa