Hello Rini,
Main bearings don't cause a knock. A bearing causing a knock will be obviously worn. I do not think you have found the location of your problem yet. It can be other issues as Dan has suggested.
Still sounds like a rod bearing to me. If you pull the plug wires one at a time while the engine is running the knock may go away when you get to the bad cylinder.
Oil travelling from the oil pump goes through the oil filter and then to the main bearings. In turn each main bearing supplies one or two rod bearings with oil. As a main bearing wears, it leaks oil and starves one or two rod bearings of oil. These rod bearings overheat from lack of oil and fail. Excessive clearance between the worn bearing and the crankshaft, as the piston and rod assembly move up and down, is what causes the "knock".
At some point in time low or decreasing oil pressure will become evident. How is your oil pressure?
Other sources of knocks, as Dan has indicated, can be piston damage or wrist pin problems. Valve train problems are other sources of mysterious knocks. With these items, oil pressure is not effected.
One indicator you have given is that the knock occurs "under load". This is normally an indicator of a rod bearing problem. You need to find the problem before yous start replacing parts.