If it is a rapid knocking noise, it could be the Oil Feed Pipe, that comes from the bottom right of the engine, then up the back by the bulhead, these are notorious for breaking down internally and causing a knocking noise on start-up. Alternatively it could be Variator noise, one at the front of each bank. These are fed by oil held under pressure in a tank situated in the centre of the "V" "Accumulator", if this loses gas pressure then there is no pressure to push the oil to each variator on start-up, until normal pressure from the engine get's there. Get someone to start the engine from cold while you are under the bonnet, see if you can locate the noise..!!