. . . Yesterday . . . a strange sound coming from the engine. It sounds like a chattering / rattle when the throttle is pressed. The noise happens both in gear and in neutral. It sounds like an exhaust leak . . .
Hi, and welcome to the forum.
An exhaust leak can be diagnosed by blocking off the tailpipe with the bottom of your shoe or other suitable stopper. A tight exhaust will hold enough pressure that the engine stalls. If there's a leak, you'll be able to track it down by the sound once the main opening is silenced.
It's hard to say much based on the recording. My first thought was piston knock, but you would expect that only under load; Not in neutral. It could be as simple as a loose part rattling. If you can recreate it in the driveway, it might be worthwhile to put the truck on jack stands and crawl under while an assistant revs it. Sometimes rattles are interior parts, like the instrument cluster glass. It's always worth touching things with your finger to see it they change the sound.