Probably vibration and the like. I just replaced my transmission mount as it was completely trashed. I removed the bottom bolts and it just fell out, and it still had a bolt on the top. Now I cant say if this is exactly what helped cure my vibration as I also replaced both flex discs and my center bearing at the same time, but I'm sure it helped out with the vibration.

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1984 300SD Orient Red/ Palomino
1989 560SEC
2016 Mazda 6 6 speed manual
1995 Ford F-150 reg cab 4.9 5speed manual