Seller provided many photos of the rust coming to the surface. Also a strange "driving" video - To me is the motor sounds to be missing a cylinder. Maye a shifted or broken rocker arm making a loud ticking noise (hence the inline 5 noise).
bringatrailer.com

Euro 1969 BMW 2800CS 4-Speed
Bid for the chance to own a Euro 1969 BMW 2800CS 4-Speed at auction with Bring a Trailer, the home of the best vintage and classic cars online. Lot #182,853.
