could be that long bolt that the alternator pivots on, you need to loosen that when changing those belts to drop the alternator down, i forgot once or twice to tighten that back up myself. Did it only appear since the belts were changed?
if its the front o2 sensor nearest the turbo, which is common to go, its 330 euro from subaru, can be gotten for about 70 euro or so from Japanparts i think