Has anyone here had the problem of their Sporty sounding like a tractor?

The lifters have slowly developed noise and now it is annoying the bejesus out of me!

I had a similar problem with a ute that I used to own, and it was the lifters hitting the inside of the rocker cover. However, when I replaced the Rocker Cover Gasket I noticed that there is quite a lot of clearance.

This has caused me to wonder if it may be the oil pump is not working properly, however looking on the Kia tech info site it appears that replacing the oil pump is quite a lot of mucking around which makes me wonder if it is worth the hassle, or should I just cheat and run a bottle of Moreys through it?

Anyone have any suggestions? I would appreciate any help I can get!