Glad you resolved the mystery. It was another forum thread that brought to mind a "related" mystery.
https://e9coupe.com/forum/threads/heartbreak-broken-rear-shock-mount.18208/
Many years ago, we had a curious encounter with an E9'r trying to diagnose what he thought was an abnormal engine noise. On an early Sunday morning drive, when few cars were on the road, we noticed a beautiful Polaris E9 being driven so that it was swaying from one side of the road to the other. This seemed deliberate rather than someone who had little control over his faculties. He eventually pulled to the side of the road and suspiciously circled the car. When we approached him, the driver explained he was hunting for the source of an abnormal engine noise. I did not hear the noise, but evidently, the noise seemed loudest in some but not all gears and did not seem related to any specific driving style. When the driver retrieved a flashlight from the rear of his vehicle, I noticed what seemed like unusually tall protruding threads at the top of one shock absorber. This was followed by discovery of (easy-to-overlook) jagged hairline cracks on both shock towers.
At rest, there was nothing obviously wrong. However, both towers were kaput. Suspension travel, given the right circumstances, permitted the exhaust to contact the body. The resultant sound evidently resonated so that it was mistaken for an engine noise. Possibly worth noting , the E9 was apparently fitted with extra firm KYB gas shocks. Whether this caused or contributed to the problem is unknown. Due to age and no vehicle history, the problem could have preexisted latest shock installation. (One of my cars experienced a similar failure with the original extra soft Boges. The owners manual never specifically warned against railroad track crossings and other road hazards.
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