ohh dear; you dont have much luck do you mate? - you havent had it on the road long!

Basically it sounds like big ends from the description you've given; big ends tend to make a hammering noise when under load (e.g. not idle)

if you rev the engine between 2-4rpm you'll "most likely" to hear it then. Another is to try and drive up a big hill in a high gear, it will be noticeable, but it will depend on how bad they are to the noise they make

it could be piston slap but i doubt it, as piston slap normally goes away once warm - having said that mine sounded awful for the first 5/10 minutes

whats the history behind the engine? has it been refreshed?

best thing to do is post a video of the motor running. you can rev it up from under the bonnet

matt

if you were closer i'd have a look