Oil level is normal. What does the condition of the spark plugs have to do with this? It is not running rough.
What is a leak down test?

I have the exact same issue. Parked the car one night, the next day it makes the same noise as in the video by volvo_96.
Does anybody have any ideas what it could be?

I had the same problem,
I look under the seats an noticed the wire was broken towards the seat.

You might have major engine work.
Broken or belt valves if you timing belt broke while driving.

Sometimes it works and sometimes it doesn't.
What do I have to do to change the bulb in the trip computer?
Do I have to remove the cluster?

'96 850 turbo 2.3 20v...about 300k km and still runs great!

There is plate that wraps around the bottom of the crankshaft.
It is held in place by two 10mm bolts.
I had to remove this to get my belt off.
The end of the two bolts are very close to the timing belt.
About 1/16 of an inch. Is this normal. I guess I am worried that the end of the bolt might scrape ...

When I open the driver door, the driver door light doesn't come on. If I try any other door it works. Is there a switch in the door that turns on the lights when I open the door?

The bolts that hold the vibration guard in place.
THe bolts only have a bit of threads.

I guess nobody noticed how close these bolts are to the timing belt!