The http://www.volvospeed.com/bay13.htm is a great site to start
with. An oilleak at the oil return line on the turbo is a common
prblem. Figure a couple dollars in parts and one hour labor to fix.
Your mechanic should not have to open the turbo -- the turbo itself
is very reliable. In fact, I would not let him open it -- better to
get a rebuilt one from someone who rebuilds htem all the time (but
really, that probably is not needed). If he insists, have a dealer
check it out first.

My 98 had a thump when I went over small bumps. It was an
exhaust hanger banging the underbody. Tapping the exhaust pipe
after the catalytic converter with a rubber mallat would reproduce
the problem and I found where the hanger was hitting. Not likely
your problem but something to try.

I have Michelin Pilot XGT H4. This is the thhird set of tires
on my 98 T5 and the ones I like the best. Others were too harsh or
too noisy. No tires have lasted very long though. Figure 25K miles
out of a set if you don't drive it too hard. Also, I live in a
mild climate (no snow, little rain) so this tire works for me but
may not be the best for your location.

Noisy rear shock is not that common in the sedan. Some of the wagons
had issues with this. Try lowering the rear seat backs while you
drive to see if that helps to pinpoint it. Also make sure the spare is
bolted down tight and not making the noise.