Q: Car has Knocking at startup... Sounds like a diesel 2011 Hyundai Elantra

Hi there - your engine, like many others, uses an oil-pressure operated mechanism, sometimes called a cam phaser, to vary the camshaft timing (aka variable valve timing). This mechanism delivers additional power and fuel economy for more recent engine designs. The cam phasers use engine oil pressure to operate, and can leak down when the engine sits for a while (the 3-4 hours you mention is enough) or overnight. The result is some metal-to-metal contact when the engine is first started, until oil pressure comes up, and cushions the mechanism. If the noise seems excessive, I recommend a noise inspection performed by a mobile, professional mechanic, such as one from YourMechanic, who will come to your location, diagnose this problem, and give you an accurate assessment of damage and cost estimate for repairs.