Idler and tensioner pulleys will do this. Pop the belts off (check them for cracks in the ribs while they are off) and spin the pulleys by hand. Bet you find one or three that are really loose (they shouldn't freewheel spin, there should be some resistance).

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Kirk
Las Vegas
2016 X5 40e Mineral White/Black Dakota Leather, ZLL, ZCW, ZDA, ZDB, ZPP, multi contour seats, rear side window shades, HK stereo
2011 E90M3, 6-speed manual