Well, it certainly shouldn't "CLUNK" Anything that sounds like a bag of hammers needs to be investigated until it's fully understood. Clunk, clunk, clunk leads to BANG, then silence. Then an excessive amount of words I can't write on the site.
Pull the valve cover and have a good look at the tension roller down in the middle of the cylinder block. Not easy to see but i would be looking for missing roller rubber. 40 year old tensioner rollers really should be replaced no matter how good they look. How many times have we seen someone report a damaged tensioner roller and then bent valves.
Manual tensioners are not for everybody. I would say that they should be checked every 50 hours running time until you get a sense of how often they should be checked. I've seen some engines needing checked every 20 hours and some looking good at 200 hours. You'll know by how much you need to adjust in the adjuster. My advice would be to adjust out the slack while running at idle then turn in the adjuster bolt one flat (1/6th of a turn in) and lock it up. The chain needs to be tight enough to keep the tensioner rolling and not allow the chain to slide over the tensioner roller.
So let's hope you've caught it in time. How many times have we heard that someone was running a bike with strange noises and kept running it trying to "figure out " the problem until BANG/ silence. And then the famous line "Oh, I shouldn't have done that"".
Good luck and keep us in the loop please.