Take a look underneath and make sure it isn't a flex disc starting to fail- especially under load.

I'm sitting in San Diego as I type this- 1500 miles away from home and I just got my car back it did the exact same thing- except it failed on the highway and almost dropped the driveshaft. Had to pull over and find a shop.

rjp

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03 540I (GAK!!)
81 280E (W123)
70 LeMans Sport with pissed off Poncho 462"
1989 Silverado Step Side (under the knife- complete restomod)