I got this bike recently at 6k miles and have done 4k on top. It only had one owner before and never had chain and sprocket changed. A month ago I noticed a chain rattle when pulling from first gear that goes away when I tighten it to the tight side of the slack range. BMW says leave the chain slack between 35mm and 45mm. The slack went away when I set it to 35mm. I then noticed a quick wear of the chain and now it rattles even at 35mm. The chain is still within it's recommended 144mm (across 10 links). My front sprocket is looking a bit worn so I tried to fit another one to see if that was the problem. I still intend to replace both sprockets' and chain in a couple of weeks. All the links move freely and none are seized. With the chain out of the way and the sprocket bolt removed, it is supposed to slide right off. But it is stuck. It looks like it fused with the countershaft spindle. I have tried soaking it with penetrating oil with multiple applications for over a week. Tried hammering it, tried to dilate the sprocket with a blowtorch even. I tried a gear puller but it still won't budge! [Here is a photo](https://i.imgur.com/cZvt4ty.jpg) of how it looks. [Here is a video]() of the rattle. And [here in slow motion](), with the knock happening at 00:32s. I don't want to get a new chain and sprocket set until I can get the old sprocket off. Until then, what is causing this knock and rattle?