its not just 1 clunk though, it continues till i put my foot back down. its an identical feeling as trying do 10-15mph in top gear going uphill (what i call kangarooing it for some reason) but without the nasty wiplash. there is some really light jolting while this happens but its not uncomfortable like the engine is about to stall. its really hard to describe here though
this is what makes me lean towards the gearbox mount at the moment, to me it seems like the engine is unbalanced and when at low revs its enough to rock it back and forth making the noise