Thank you everyone. That's what I needed to know. Interesting that there are two different opinions as to which disk is the more likely to go bad. I do believe that, in my case, it is most likely the rear one. I will probably get under there and get the rear changed right away, and take a look at the front and the bearing etc.

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Andy_M
'98 W140 S320 Black/Black
'06 W164 ML500 AMG White/Grey
'03 Porsche 986 Midnight Blue/Grey