Caution on the dealer, was good advise, warranty yes, good mechanics doing the work, no. AS mentioned above, a lot of things can cause the problems you described. I am completely sure, it isn't a "tappet" noise you are hearing with a 13k mile car. Rough idle, do the throttle reset a few times, this is easy and hate to say it, but the crankshaft position sensors will cause rough idle, rough lower rpms. A vacuum leak will cause the same symtoms, so check all the hoses. Throttle bodies have been known to have vacuum leak around the base... There are several threads on here dealing with all these problems. Good luck on your search.