I have a 97 XJ with less than 60k miles on the engine. Yesterday out of the blue it started making an odd ticking or knocking sound under the hood. It is much louder than the typical 4.0 tick, and only happens when my foot is completely off the accelerator while driving, or when it is in park/neutral. When I was returning home, I noticed it stopped for the most part when the engine was warmed up, and I was going 55/60mph, but it would still make the noise occasionally at speed, and when I was going slow through town.

I've isolated the sound to the very back of the valve cover, and it does sound like metal tapping metal. I'm not sure what I would need to look for, so any suggestions would be great. Thanks.