What would make this sound?

[andrewwynn]

You can use a dowel or large screwdriver vs. stethoscope but I would do one simple test first.

Stop the fan from spinning. When engine cold it doesn't take much force I do it with my fingers but I recommend a piece of foam outer rolled up paper or cardboard.

If the chip changes is the fan clutch.

The uncommon source is the harmonic balancer and those are costly so let's hope it's not that.