You say your have determined the sound is from behind the dash but I decided to respond with my somewhat similar experience.
I had a buzzing sound that came on in the 30mph range in our 89 740. I eventually found that it was the stainless trim that surrounds the windshield.
I eliminated the buzzing by inserting a thin piece of plastic (cut from a parking pass that hangs from the rearview mirror). That "loaded" the edge of the trim and stopped the flexing that was allowing the buzzing.
Randy
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Any twenty minute job is just a broken bolt away from a three day ordeal
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