I had this problem with my Subaru a few years ago. I'd get an intermittent horrible crunchy spinning noise from the front of the engine. I thought the alternator or AC compressor was going. Pulling of one belt at a time then starting the engine briefly narrowed it down, then exposed the tensioner pulley as being the culprit. Removing it, I couldn't believe how bad the bearing was, like it was filled with gravel. I'm actually surprised it still spun without eating itself. The replacement from Gates was all steel (OE was plastic) and had a larger bearing. It ran SO much quieter after that. One of those rare easy fixes that made a huge difference.