Big end bearings would cause a knocking noise where as main end bearings would be a continuous rumbling sound.
Try to establish what speed the sound is, ie, engine speed or half engine speed. If it is engine speed, then I would say big end bearings, but if it is half engine speed and you say the sound is coming from the top end of the engine, then maybe look at tired tappets, although these would not knock, they would be more of a clatter.