“A loud knock noise” is too vague to diagnose, but it could be a hydraulic lash adjuster which has collapsed causing a knocking (tapping) at start-up but that re-adjusts a few seconds after it receives oil pressure, or it could be a wrist pin, rod, main bearing or some other reciprocating part within the engine.

Several years ago I had a Mazda van which when the engine oil had about 2,500 miles on it several of the lash adjusters would collapse causing a tapping noise for several minutes after start-up. Within 100 miles after changing the engine oil the lash adjusters would again function correctly.

Additionally, also with the Mazda, it developed a knocking sound that I would have bet my life was either a failed rod or main bearing. My mechanic son-in-law, who also initially thought the same thing, put his stethoscope all over the engine and determined that the knocking was coming from the power steering pump. I replaced the pump and the noise was gone (the power steering pump is a “vane” pump, and I still don’t quite understand how it was making a very loud knocking sound).

So, your knocking sound is going to have to evidence itself to a mechanic which may be problematic (you might consider tap recording it for play back to a mechanic).