Hi mate,
It's coming from what I can only describe as the middle of the engine, it only happens say 2/3 times in 10 and it's only for a second after ignition, it's an automatic so no clutch. I need to really get another person to start it with the bonnet up but with it only happening sporadically. It sounds to me as if it's something winding up and when the engine reaches it's normal idle rpm it goes completely, hope this helps