When I start my car in the morning, engine make bit louder cracking noises like something rattling or some iron hitting to iron sounds. But within 3 minutes, gradually engine sound gets back to normal. It happens only on cold engine.
My car has 174000 miles.
My car has an automatic transmission.
Hi, most likely, this noise comes from worn internal components in your engine that momentarily do not have oil pressure to eliminate metal-to-metal contact. When oil pressure comes up, usually within a few seconds of engine operation, pressurized oil prevents this contact, and the noise disappears.
Specifically this noise could come from a worn timing chain "slapping" the tensioner or guides, worn connecting rod or crankshaft bearings, worn valve lifters, and so on.
Given your mileage, there are multiple possibilities. To verify there is no immediate problem, I would recommend having the noise in your car inspected by a mobile, professional mechanic, such as one from YourMechanic, to diagnose this problem, get an accurate assessment of damage and cost estimate for repairs.