I had something that sounded similar on my trophy. It eluded multiple mot testers and investigations...
I had the drive belt tensioner and guide wheel changed the other day and the mystery clunk has disappeared!
I had to travel from the south east to west wales for mine! It is worth it financially to buy one what has been looked after and doesn't need much work. I have spend £1,500-£2,000 on mine since I bought it getting it up to scratch! Would have been cheaper to buy a more expensive one that had had...