Take the belt off and spin each idler bearing by hand, feeling for any roughness and listening for bearing noise. Same with water pump and alternator.

I also know that if the port for the big breather hose on the driver side valve cover is clogged, the engine builds up internal pressure, which causes weird noises as the pressure comes out.
I thought I had a bad water pump.
You are all probably wondering how I diagnosed that. Simple, I caused it.
I thought that hose was unnecessary, so I removed it and capped off both ends. Don't do that !