My 4.6 V8 was rebuilt at 165k, before my ownership. There's a constant tapping sound regardless of engine temperature when the engine is on.
I did take it to my mechanic, and he suggested it was the camshaft.

I've lived with this for 8k miles now, but I'd like to know if there's anything that I can do to suppress the sound. Upon checking online, there are only two options that I can think of:

1. Liqui Moly Hydraulic Lifter Additive - do I have nothing to lose here, or is this as bad as placing Radweld in the coolant tank?
2. Oil change - since the rebuild, it has 20w50 Valvoline VR1. I have seen online forums discussing what oil to use, and this is mentioned, so perhaps I can stick with the same one and give it a clean flush? If not, do I go back to the recommended 10w50? https://www.rangerovers.net/threads/rr-p38-4-6l-oil-type.366660/

Of course, if the camshaft is knackered, the above won't do much but I thought I'll try the cheaper options first.

20w50 might help or might not.

It depends where the tapping is coming from. Yes it could be the camshaft or tappets worn, but also blocked oilways, worn rockers, etc. If it started 28k miles after the rebuild, it raises some questions about whether they changed the camshaft, etc ? Could also be low oil pressure due to worn oil pump, or weak pressure relief spring ? Do you have paperwork listing what was replaced during the rebuild ?

After I rebuilt my 4.0 it has been a bit noisy at start-up for the 3000miles since the rebuild, but the noise stops after a few mins once the oil gets around the engine.

You could take the upper & lower manifolds off, and then be able to inspect the camshaft, without dismantling any further ? For the tappets, you would need to unbolt the rocker shaft & then take each one out to check it.