Our 745T has begun making a clattering noise like lifter noise only under load. It's usually worse when cold. The car has 231k on it. We got it with 137k and are reasonably sure the head gasket has never been changed. There are no other typical head gasket symptoms like white froth inside the oil cap or bubbles in the coolant, coolant / oil consumption or loss of driveability. The frequency of the sound indicates that it's coming from a single cylinder. What kind of head gasket longevity can be expected on a reasonably maintained B230FT? Could a clattering (loud ticking) noise under load more likely be an exhaust manifold / gasket leak if no other symptoms are apparent?

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D '90 745T 229k, '88 744GLE 129k, '96 328i, '73 911T 100k+