I had a replacement engine fitted on my 1250. I would strongly recommend attaching a GoPro to the handlebars and recording the noise, then contacting BMW UK (preferably via X as they respond quickly) and have the dealership inspect the bike. If they can’t diagnose (which they can’t, as it’s a rare fault) then say it’s unsafe to ride a bike with an undiagnosed fault and you want something done about it. Others in this thread are happy to live with the issue, I wasn’t, and I have a new engine in my 20,000km bike!