E53 motor mounts are very prone to failure. They are oil filled, and once the oil springs a leak, the mounts collapse. Usually somewhere north of 80,000 miles, especially if the vehicle is driven in an area with crappy roads, or by a heavy footed driver.

My guess is that a collapsed motor mount was ignored, the end result was undue stress to the mounting. I know on my 2002 4.4, a collapsed motor mount caused the bracket that bridges to the block to crack. Lucky for me, it was an inexpensive part. Unluckily, the A/C compressor had to be removed to change it.

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2014 BMW 328i Xdrive
2011 BMW 335i M-Sport
2008 BMW X5 4.8i Sport
2000 BMW 528i 5sp