The whining is probably NOT the tensioner, but the airpump. The airpump is part of the emissions control system, and it's designed to preheat the catalytic converter to reduce emissions. On startup it whines for a minute or two, and may at other times come on intermittently. Your operator's manual should have a description of this system.

As to the TRACS/ABS controller: do both ABS and TRACS lights come on? Assuming you have TRACS and it's just the ABS light, your problem may be related to a fouled or damaged sensor. If both lights come on it may very well be the controller, which I understand is a pc board mounted beneath the ABS hydraulic pump block (under the hood). My guess is it is a DIY job as most of the $350 repair price I paid was the part. The board develops microcracks in the soldering resulting in broken or intermittent connections.

More information can be found at Bay 13: http://www.volvospeed.com/Repair/abstrax.htm
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(98 S70 T5SE misc mods, mostly lighting) (92 940GLE)