So you just press start, the engine starts, and you count 1-thousand-1 and clunk?

need to isolate where the clunk after 1 second is coming from. any ideas?

My thinking is it is perhaps related to transmission? Alternator?

FYI, the gasket on the transmission-pan-filter was leaking on mine, and it was clear
in it's 107k miles this had never been replaced-serviced. Thankfully no metal in the fluid but fluid was very brown and I bet the filter full and I would say people should service the trans every 50k. IF you can find a GOOD service person and you are very
careful to use the right fluid I would advise this. I would be good to see no metal in
the fluid.

Also as far a engine running, the engine should run smooth, it is possible your spark plugs are very bad.

It took 4liters of zf fluids to change what was in the pan-filter leaving maybe 5 liters in
the rest of trans which could not be accessed-changed. Changing all the fluid at once would be risky anyway as it could cause the trans to loosen a clot and in essence have a stroke! The oem fluids is very expensive I paid $22 a liter at rmeuropean.com and bmw gets $60? I found a online transmission technical doc that indicated the ford mercon sp type atf was the same, I will get a bottle of mercon-sp for $8 and find out before I again change 4liters at maybe 25,000 more miles.