Found out this weekend that 6 of my studs that hold exhaust manifold to head were gone. It became painfully obvious when the # 6 pipe came free and I thought my muffler had disintegrated (LOUD!). Point is, this is apparently pretty common (also happened to the other E28 M5 in town) in at least S38 engines, the studs work their way out over time. Who knows how long I was driving around w/ only one stud on #6, and #1 was missing 2 studs. So get a mirror and look in there. Replaced them last night, not too tough.