Ok,

 

I've looked

 

When did the noise start, and what was the last thing done prior to the noise appearing?

 

That I can't tell you..

 

For giggles, remove the serp belt and run the engine to be sure the noise is still there.......yes the alt won't be running but the wp will so just don't over do it but the car will be fine without the serp belt for a short period of time.

If it is still there....well then you know it is behind the covers and they will have to come off.

There are two "idlers/tensioners" there, inspect those for hard spots. Also there is a washer" on the crank where the concave should be facing out - is it?

 

Mine had a humming noise around the TB area. The sound is pronounced when one of the inspection plug on the TB cover is removed. It turned out to be the timing belt. The sidewall of the TB belt was rubbing and there was some metal showing on the sidewall of the belt. I don't know why the sidewall rubbed off. At that time, the TB only had ~12K miles. With the new belt on, the noise went away and hasn't come back yet.
Hope this helps. Good luck.

 

That's what I'm worried about

 

Ensure the timing belt guide plates on the crank gear are not installed backwards. Installing them backwards will result in the plate rubbing and tearing the belt up. There are two washers on this pulley. One behind, and the other in front. The tapered edge of both need to point away from the belt to prevent damage. If this is the case, when you disassemble the timing components you will notice a removal of the zinc plating on the washers.

A lot of people screw this up.

 

There was only one tapered washer

 

It could just be a "bad" belt. It could also be from a "wobbly" pulley. Over time, the belt edge gets worn down from an imperfect pulley movement.

 

How worried should I be?

 

I love it when people hijack my thread,lol..

 

If you don't want to take it apart again just keep paying attention to the noise. If it gets different or louder, it's probably need replacement. For me, I didn't want to hear the noise and since I opened it up already, I just replaced it. At that time, I had driven ~5k miles after the noise started.

 

Since you already opened it up, just take off the belt and inspect it carefully. It's hard to tell from the pics. I didn't catch the rubbing the first time I opened it. My suggestion is to take the belt off and replace it with a new one so you can eliminate the TB as a cause. After you take off the belt, inspect the other components (pulleys, tensioner, guide plate etc.). If the noise is still there after you put it back, then it's something else.