firstly the correct name is Main and Big end bearings, the main bearings are which the crank spins on and the big ends are the big ends of the connecting rods, the other end of the connecting rods are called small ends which are called gudgeon pin.
If your engine has cold start rattle firstly check that you are using the correct oil filter for your engine with a working antidrain back valve, using incorrect filters will cause bleed down and rattle.
To correctly diagnose main and big end bearing failure, you are supposed to do an oil pressure test with the correct lubricating oil in the sump and the engine at correct normal operating temperature.
If your oil pressure spikes on startup and then falls to below 10 psi pressure at normal temperature, and does not rise correctly when revved the clearances between the bearing journals is causing loss of pressure and eventual wear on the crank
Such noise is also caused by ommitting the oil control orifice in the block which regulates flow volume between the block and head (if OHC engine) - What you posted is an acceptable practice only if the top end is certified to be good. I mean checked with tools not dancing around fire, voodoo, andaaza, noise etc etc.
I can give you a rattle example of old 84-87 Honda civic 1.2 engines, they require an oil filter with a pressure relief valve which means that unless the oil pressure is high enough the oil filter will not allow filteration, this was done by Honda to ensure oil gets to the crank oiling holes immediately and when the entire oil system is full and the main oil gallery starts to pressurize the filter opens up and allows oil to be filtered.
A mitsubishi filter (cheaper compared to Honda) also fits perfectly but lacks this function, this causes the crankshaft to rattle on startup, and your mechanic then tells you that engine "over all" required. :).
Think wisely - there are factors in Pakistan that do not make sense anywhere else, they are caused by our love of shortcut, bypass, jugaar etc etc.