Cat issue (so to speak)
I had the random, tinking, underhood sound you might be describing, and various, including catalyst, codes were logged on my newly-acquired '00 X. It kind of sounded like it was coming from the head. Lying underneath the car, using a stethoscope, it sounded like it was coming from the rear O2 sensor.
The tinking sound was caused by melted and re-frozen catalyst rattling around in the passenger side cat. Here's a shot looking up into it at what I found. I broke this lump out and the sound and the code went away (well, I had to clear the code, but you know what I mean).
I'm sure the catalyst will melt like this if a bad mis-fire is left untreated too long. Unburnt fuel burns in the converter.