If they are new they can take a few miles to quieten till they get fully pumped with oil, they should really have been new ones with a new cam, unless they are mechanical ones that need adjusting because it has a wild cam, though would not have thought so.
They can often have a tappet that takes 15/20 miles to quieten fully, it's nothing to do with wearing in as such just oil pumping into them.
If they were not changed it would be a good idea to anywaym as still 18 years old, time will take it's toll not just mileage, you could try a flush to get rid of the varnish from the inside of the engine that could be causing 1 or more to stick a touch and so be noisy.
tabetha