Sanduleek

I have a 2009 C350. Last year I started hearing whining sounds as the speed got higher. After checking this out I was advised that it was my transmission bearings. The car had about 75000 kilometres. Had a used transmission installed for three thousand, which seems to have solved the problem so far, but what gives? Transmission bearing problems at only 75000 kilometres? Should I get rid of it now or wait?