Metal-on-metal sound near water pump/belt areas?

What is your question?

I had the water pump, timing belt, spark plugs, and all gaskets changed 6 months ago. Two months ago a rather loud and annoying metal-on-metal sound has been present when the engine is on. First thoughts were a belt, a roller, a bearing, maybe the water pump. Mechanic took the engine apart and changed a roller, but everything else is checked out and running perfectly. He cannot pinpoint the source of the sound. To me, it sounds like a belt or a roller is causing it, but it's not. It does not "accelerate" with the engine, it just is almost like a very loud constant noise. It's annoying at best, but if I don't find the source I'm just constantly driving around thinking my car's gonna break at any minute. I don't need that stress. Any ideas?