Record the sound and post it up here, I bet it's actually the classic exhaust manifold crack that our cars are susceptible to. Although if your mechanic is any good at all he would have been able to tell the difference.
As for valve issues. You can adjust the cam lobe to lifter clearance but you do it by replacing shims. I would just move up to one level thicker oil. i.e. if you're using 5w20, go to 10w20 / 10w30. (On older cars it's not a huge deal because the clearances aren't as tight, but yes, if you put 20w50 in a brand new car that needs 5w20 it's going to be toast.)
An easy solution is to try adding some Lucas oil stabalizer.
Also, it's not a big deal... If it's a quiet tick. If it gets really loud, then worry about it.