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It's the multiple pulley device on the bottom of the engine in the front. The belts from the power steering pump, alternator and ac compressor all are driven from this unit. It's very tightly connected to the front of the engine crankshaft with a big bolt. On the Volvo the outer and inner pieces are seperated by a section of thin rubberlike vibration dampening material. Over time the rubber bond deteriorates and the inner and outer parts start to shift in relation to eachother. This makes it very difficult if not impossible to correctly set the ignition timing because the timing marks are cut into the outer section. Mine started to squeal alot every time I'd turn on the ac compressor. This drove me nuts trying to fix the compressor because I thought the clutch was slipping.