Jim is right, I'd describe it as something between a loud hum and a roar. The noise should change character as you zig-zag, loading the bearings from side-to-side. A bad bearing doesn't necessarily result in axial slop unless the bearing has advanced failure. If you're still unsure, you can always regrease the bearings and perform an inspection at that time.

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95 E320 Cabriolet, 169K