I recently rebuilt a second engine for my 98 Discovery, everything went well and then engine runs great.

However i noticed now that when i accelerate there is a ticking noise that seems to originate from the rear of the engine compartment on the passenger side. It sounds like a metallic clicking like something hitting the spokes of a bike, and sometimes can get rather loud.It starts at about 2k RPM and increases with RPM until i let off the gas, it shifts fine and there is no loss of power. It only happens when the transmission is in gear, it will not do it in P or N.
So far i have checked....
Changed right side exhaust manifold and gasket and made sure everything is tight.
There are no exhaust leaks.
There is no loss of oil or Trans fluid
Flex plate is solid and tight
Spark plus are tight
Both drive shafts are good-test drove with both of them off, noise still there.
Rockers, pushrods, and tappets all good.

This is driving me insane, please help.