OK, armchair mechanics, I need insight to triage a problem. 2009 XF US Spec, 120K miles. Rear end on this car has always been noisy, a hum from 5 mph- 50 mph. Last 2 days, there is a loud scraping (as if brake is frozen) and metallic clank when starting out, noise dissipates back into it's normal hum after driving 1/4 mile or so. If car stops after driving a bit, scrape & clank is minimal when setting out again. After car sits for awhile, though, the pronounced scrape & clank returns. No brake warning light showing, no real feeling of drag when driving. Thoughts?

 

Not really but have you drained the fluid to take a look?
Obviously if you see metal you have your answer.
Have you checked the level in the rear end?

These are free to do before spending any money.
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Possibilities include brake problems, wheel bearing really going bad, transfer case or propshaft. Could also be the transmission. My money is on the transfer case, which could be a big deal if it turns out to be the issue. Drain the oil in the case and inspect the oil and magnet for filings. PS, make sure you can get the filler bolt off before you drain the transfer case. I had to replace one wheel bearing at 66k so if you haven't done them yet, you may be due...