Sonata NF (2006-10) :: 2007 3.3L V6 With Intermittent Code P0018

2007 with 3.3L at 30k miles (bought new and with regular oil changes) starting throwing p0018's intermittently. No noticeable difference in way the car drives: idle seems fairly normal (only once recently felt the idle briefly stumble at a red light), fuel consumption regular, no power problems. There has been a engine tapping sound at low speeds only heard until the engine warms up after a couple of miles (I'm in Florida, temp is regular in the 70-80's), and the sound is not evident during the start-up, only while accelerating slowly.

On occasion, with hard acceleration, the "ESC 0ff" light comes on, not affected by the ESC off switch, and stays off after one key turn.

Brought to Hyundai dealer who, same day, requested doing an oil/filter change (Firestone had done a change 900 miles/3 months prior) claiming that a wrong oil filter could cause blocked oil galleys. They did the change (and also, no charge, repaired a valve cover leak) and 6 days later called to tell me the car was fine. No mention of the car having been low on oil or evidence of sludging. Also, no mention of whether or not the filter that was replaced was the incorrect filter for the vehicle. (I called Firestone and they claim they regular work with Hyundai to ensure proper filters are used). No mention of why they kept the car for an additional 5 days after the oil change before calling either (except for repairing the valve cover leak with a rocker cover gasket), and although their invoice indicated that they drove the car 5 miles, it was evident from the invoice that the mile in/miles out were the same. I did notice that there was about 2-3 gallons of fuel consumption though when I picked the car up.

6 miles from the dealer after pickup (2 key turns) CEL returns, with same code. I have now driven the car about 100 miles in the last 20 days with the CEl going off and on 4 times now, and each time the same code. I have never had the CEL come on (or go off) while driving, only at start-up. (Longest trip has been 10 miles, top speed 50mph, so a lot of key turns). The longest the CEL has stayed off was for a 30 mile span. The tapping sound mentioned above continues too.

I'm hesitating returning to the dealer. The 10 yr warranty might be a problem for a variety of reasons, and I realise finding the fix for this code could get expensive. But I do want to know what the dealer found out doing their GDS (and via eyes on inspection) and nothing was mentioned on the invoice besides the gasket leak. I'm especially concerned about the timing chain and tensioner.

Is it acceptable to make an appointment with the service manager to find out what they ruled out (and then take that information to a regular mechanic)? Given the problem is regularly intermittent over the last 100 miles, is it still possible that something might be seriously wrong with the engine and damage might be done to the engine by driving it? When I first brought the car to the dealer, they said they couldn't do anything with it because the CEL had gone out. Fortunately, within a mile from the dealer the CEL returned and they took it in. Presently, the light is out again.