Is there a good Independent workshop/mechanic you can ask to listen/advise, it shouldn't have to be a Fiat specialist, just someone with a bit of mechanical ability after all it is just an engine, what ever make?
You may be able to locate it using a piece of rubber tubing/fuel pipe held to your ear and directed close to different parts of the engine.
In general timing chains rattle where as valve gear is more likely to to tap, either way the pipe should guide to a general area.
If valve gear tapping that comes and goes, it can be a worn cam follower as they are designed to revolve to even out wear, so sound may come and go a bit and also just be one that is causing the knock, although if repairing it is normal to replace all of then as a set along with the camshaft.
If more serious knock in engine, if piston wear it would be noisy cold and possibly reduce as warmed up and metal expanded.
If engine bearings usually noisy when first started before oil gets up to pressure and fully around engine, however if oil hot and you had a noise under load with higher revs that also can be bearings, which quietens as you take your foot of the accelerator.
I assume correct grade of oil, and filter recently replaced?
I have had engines that leaking carbs watered down the engine oil causing bearing rattle, though often with experience you can "sniff" the dipstick and smell petrol/gasoline in the engine oil
