What is the rattling noise coming from underneath when truck is in gear?

Hello, the rattling you are hearing sounds like a heat shield on the catalytic converter that is coming loose. Heat shields will cause harmonic buzzing noise as the exhaust vibrates in concert with the running of the engine. A trained tech can easily spot this. Good luck!

Truck parked with brakes on - engine off ... Crawl under truck > using a rubber mallet bang on exhaust pipes to see if noise is present .. Don't forget about the catalytic converter as well ... Broken catalyst inside the converter is common problem and will rattle like hell.