Whirrlllll...noise if i drive at steady speed. No noise if I accelerate hard or if I release the throttle at high speed. On steady speeds it's there sometimes.

Sounds like gear noise. I tested it with putting gear to N and revving up until the clutch engages. I can hear it then too.
Did inspect the primary transmission in some way. I put "noise reduction" liquid to the transmission -> no difference. I have had success with this liquid in previous cars with rear end gear noise.

Because the noise is there with revving at neutral it has to be:
1) conical gears at primary transmission
2) Clutch housing rear bearing
3) primary transmission front bearing

Because of the same reason (revving at N) it cannot be:
1) Clutch release bearing
2) Belts
3) Secondary transmission gears
4) Angry owls at the nearby forest

More ideas before I replace the primary tranny for nothing?

good thing you tryed N too, that simply just eliminates half the driveline making the search a whole lot easyer . i'd put my money on number 2, Clutch housing rear bearing.
very common bearing to start producing noise, moisture from the outside and clutch dust from the inside attack the bearing over the years

it's also the easyest bearing to test, only the propshaft and tailpiece of the bellhousing has to be removed, saves a lot of work compared to bellhousing removal

ehm...I tried removing this but the clutch shaft seems to be pressed into this bearing. It is really stuck anyways.

You can't remove a clutch in a Volvo 66 without taking the engine out; don't know the consequences for all the ancillaries.

the bellhousing has 2 pieces, front piece is bolted to the engine and has the engine mounts attached; rear piece is bolted to the front piece an can be removed with engine in place...

Yup...but how am I supposed to get the tail piece loose. The clutch shaft is pressed? into the bearing...I removed the prop shaft and the four bolts that keep the end piece in place but now what?

should come off with bolts removed, a slight tap with a rubber hammer might be nessacary