I spoke with the dealer after having my rar main seal replaced (ouch$$$) and he said the slip coming from the trans is associated with something called a grabber gear (might have the exact name wrong.) As the tech explained to me(and I may have some details incorrectly translated here) the gear is pushed into gears on the trans when you place the car into park. The popping sound you har is this gear trying to lock into the trans gears to stop the car from rolling while in park. This is apparently common and the tech took a look at my trans while the rear main was being replaced and said it looked fine and not to worry.

I picked the car up last night and gave back the XC90 AWD loaner. It is a greatfeeling to get back into your 10 year old car that holds its own against a new $45K vehicle, still gets me around in comfort and saftely with milage anywhere from 22-27 MPG and is fully paid for. The older Volvos seem better built and will last much longer than the new model in my opinion.