This question is for "s70driver" or anyone else with experience relating to Volvo AWD systems. The info they have available for these awdsystems on the internetis non-existenat or very hard to find so any info is greatly appreciated. I have a 99s70 AWD that is making a horribile noise directly underneath the car(under the front seats)when I accelerate to about 35 mph and let off and coast, or sometimes it does it much more and even when I'm just idling at 5mph or accelerating. Sounds like sombody put a thick plastic stick into a running ceiling fan and held it there and it's realative to speed or rpm's I should say. Sounds just like my accord didafter I ran into a dirt embankment pushing the fan back a half inchto the crossmember, but it's coming from right under the car. Also, I have been trying various corners atvery high speed and it seems as though the rear wheels are actually transfering power when I start sliding. Can it be true that if it detects wheel spin even after 30 mph, it will transfer power. Itcould be too that it handles sooo much better than a Honda, andthat I'm used to horribile understeer or whatever. Sure feels likesolid tractionsimilar toa subaruawd though. Thanks.

 

You could have a bad driveshaft or angle gear.
The AWD is supposed to be disabled above 30 MPH.

 

The rear AWD unit engages and disengages when it detects slipage. There are a series of disc bathed in fluidinside the unit; when there isslipage they heat and engage.

Before fixing the noise; check that your tires are all within 2/32ths of mismatched tire can cause problems in all volvo AWD systemunder -03. 02 was the switch over some 02's have the older system some have the newer system.

Like tech said the angle gear and driveshaft are possiblities. Take a look a the front driveshaft joint and were the angle gear output shaft connects to the case.

 

I have the same car and I had exactly same issue. It's from prop shaft ( Drive shaft) there is a bearing right in the center of the shaft which connect the shaft to the underbody it's been called U- joint (Look at my posting). Most likelythis bearing is shut, they are not repairable at all, you have to either put new shaft in, which cost you around $1500 CAN. Or do what I did, remove the shaft and make the car FWD.
According to the "Tech" it's a common problem in these cars.
Installing a new shaft also does not guaranty that the noise will be eliminated as you may have problem with the Transfer Case ( Bevel gear) too, this is the unit that actually transfer the Torque to the rear, you don't now if there is problem with this unit until installing a new shaft, which is too late because if there is a problem, then you also have to replace the Transfer case and it’s very expensive, $2,700 CAN.
I suggest starting with removing the shaft then test drive the car,
Let me know what happened.