I have a 2012 JSW with 2.5L. The car was purchased new. It got routine oil changes with Castrol oil and VW filter. I think I treated it well. It has been running well with no issue.
In this morning, I started the car as usual. I noticed the engine started weaker than usual. I heard loud knocking sound from the engine. I swear this was the first time I heard this sound. I knew 2.5L usually starts loud, however, this was very different noise. The knocking was so loud and made me feel the engine was going to die. I shut down the car immediately, and checked the oil. Nothing was wrong. I restarted the car. I still heard strong knocking from the engine. The exhaust gas was smelly and definitely not burned well. I decided to let it run for a while. After about 10 minutes, the knocking slowly disappeared. I took it out for a spin. It seems running smoothly with no check engine light. I drove it for an hour and did not notice any major issue. Later in the day, I did not hear that knocking sound anymore when I started the car.
A little more information. This car was not driven for 1.5 days. I did move the car last night from my garage to the driveway, which was basically a quick start and stop (maybe 2 minute). I did not notice anything weird. The outdoor temperature at night was chilly, about 50.
Should I let a shop check this car? What may go wrong?
Thank you for your help!