Hyundai Sonata Engine Problems

It's most likely to be a sticking tappet or tappets and it takes time to pump up once you start the engine. In the three days the car was sitting the oil has had time to drain back into the sump, so it will take a short time for the oil to be pumped up through the engine again. Have a mechanic listen to the engine for you to determine where the knock is coming from, or change the tappets.

It would seem to be in the fuel system, so I would be looking at the injectors to find the cause.

Valves bend because they come into contact with pistons. It could be that the belt wasn't installed correctly and the cam timing is out, it could be that the belt tensioner has failed and the belt has jumped a couple of teeth as a result, it could be a problem with the lash adjuster.

It’s generally done in these situations because it has to be removed to replace the water pump anyway, which makes it a convenient and cost-effective time to do it. The fact that it was done 35,000 km ago might make you question the need to do it again so soon, so have the mechanic check it and if it appears in good condition perhaps replace it, but I would err on the side of caution and replace it anyway.

IT SOUNDS as if you have a bent valve. The Sonata engine has to be correctly aligned when the belt is changed and it's difficult to do. The other thing to check is the balance belt -- it should also be changed, and needs to be correctly aligned when the new cam timing belt is fitted. It's easy to get the balance belt 180 degrees out.