My '99 Explorer recently began making engine noise. It's a rattle that's heard when the vehicle is in gear and idling. When it's first started, the noise isn't there (but a few blocks down the road it'll be heard), nor is it when the vehicle is placed in neutral or park. I've just started with a new mechanic who says he wants to swap out the alternator to see if that's the culprit.

Any thoughts would be appreciated.

 

I would check to see if any of your catalytic converter shields are loose. These will cause a rattle at idle and normally only appear after the converter and shields heat and expand enought to loose contact with each other. Then they vibrate against each other causing the rattle. It's a common symptom. Ford uses a large screw clamp to band around the shielsd(s).