Well, reading through all these posts, I have resolved what the screeching noise my 02 AWD S60 makes when in the snow or heavy rain, why my headlights seem so dull, and that S60 have crappy locks. All this after only owning the car for 3 months! What I have not resolved is this. . .
When the car is driven at higher speeds, usually on the highway but sometimes even around town, I hear this rattling sound coming from directly underneath the car, somewhere in the middle and towards the front. This only happens when the car is warm. It does not happen on my morning commute of 20 miles, but almost always happens on the drive home in the afternoons. This has been an intermittent problem, which the dealer has loved. We have our fourth appointment on Monday, so any advise would be much appreciated.

 

Sounds like it might be a driveshaft issue.