Greetings Everyone,

I have a 2012 Hyundai Tucson, bought 2.5 years ago from the original owner with about 25K miles on a fully rebuilt engine, and 114K on the odometer (now 125K). I have all the service records, and the transmission fluid and filter were replaced at the time of rebuild.

It had been a textbook perfect car until a few months ago, when I noticed a slight clunking upon shifting from Park into Drive, which happened only intermittently at first before becoming regular. A few weeks later, the check engine light came on.  I went to a local transmission shop, where the owner (who was in the process of shutting down and retiring) found it to be an unrelated evap system code.  He tried to reset the control module to its factory transmission defaults, but was unsuccessful.  Just after that, I had a bad injury and surgery that've prevented me from driving for a couple of months, so it's just sat there.

Because it hadn't been driven for so long, a few days ago we decided to take it somewhere with my wife driving as I'm still unable to.  Then, yesterday it suddenly began clunking VERY hard when placed into D, so hard I'm afraid it'll break something.  So, needless to say it's parked again.

It shifts quickly, smoothly and quietly into 1st from P when the transmission is fully cold; perfectly normal.  Driving maybe 3 or 4 blocks warms it up enough for the clunk to happen.

While warmed up, if it's first put it in Neutral and allowed to roll forward down the inclined driveway before you shift into D, it absorbs the clunk to the point that it doesn't happen, or at least that you can sense anyway.

This very hard clunk is instantaneous upon shifting into D with your foot on the brake, with zero delay whatsoever.  So, it doesn't seem like low pressure or a valve sticking.

It consistently happens exactly the same each time.

It happens only when shifting into D (from P or R), and not when shifting into R.  So, it at least seems independent of any internally-shared 1st-Reverse mechanisms.

Once you've placed it into D, it then upshifts, downshifts and otherwise functions 100% flawlessly at all speeds and under all conditions, whether in normal automatic D mode or when operated manually. No slipping, shuddering, stalling, slamming, freewheeling, slow/delayed shifts, too high or low RPM shift points, nothing, nada.  It simply works perfectly, like a brand-new transmission.

Placing it in Active ECO mode makes no difference, which I expected anyway.

There are no rumbles, pops, clacks or vibrations of any sort while driving.  Just quiet and smooth, exactly as it's always been.  Everything feels smooth and tight through the steering wheel at all times, and just before my injury I had the tire shop feel for anything wonky in the front end when they rotated the tires.

FWIW, the local auto repair industry in my small Arkansas town is legendary for corruption, to the point the only mechanic shop with a reputation for real honesty sometimes has a month-long wait list, ditto for his brother who owns the best collision repair in the area. The transmission shop I mentioned was the only one I trusted, after bad experiences with two others plus negative online reviews and stories from friends. It just reopened as the 2nd location of a shop 60 miles away, with hopefully the old employees, but, who knows? If anyone has an idea what may be behind the the clunk issue, I would be MOST grateful!.  If it turns out to be something minor-ish, I have a friend who can fix it, as I'm currently unable to do anything myself while recovering. Much, much thanks in advance to anyone who can offer help! 

Johnathan