My truck started this noise a dew days ago. It's not terrible in this video, but after sitting overnight and the engine is really cold, the noise is A LOT louder and lasts longer. I can hear it at warm idle too, but just barely. I'm hoping it's not a rod knock...

Truck is a 1998 Ranger w/ a 4.0L

Video:

 

I was reading tsb's on nhsta.org (google technical service bullitens and recalls, first website)... found one about this noise... say's its timing chain and something else. good luck figuring out what it is...