As said it's most likely to be the lifters, when was the oil last changed, as if this is neglected this can affect them!

But otherwise if it's been standing for a while it is normal for them to take a while to quieten down.

Worst case scenario one or more of them is blocked, sometimes a 'spirited blast' can help as the oil gets hotter and thus thinner which can help!

Or as said try the Wynns product.

Martin