I had a squeaky squeal coming from the engine bay, which drove me mad, It had a mind of its own, sometimes it never happen then next time I started up it would go on all day. All the way to Paris in fact!! I couldn't pin point the noise it seemed to be everywhere around the engine bay. Well as you've said the tensioner is pricey, so I change the belt (cheapest option) and now silence. You must go for a genuine belt though, I could only get a pattern belt to begin with, that last about 40 miles before it started to sing.