2013 Dodge Ram 1500

Is it when the engine is revving higher or when you're actually moving faster?

It does it as soon as it starts up, while idling. And the tapping gets more rapid as my speed does.

Probably just the gasket between the engine block and the exhaust header. It goes faster as your speed increases because your pistons are running at a higher rate. (My 2013 had the same problem.) If it isn't the gasket, you may have a valve sticking in the engine, which is a more extensive repair.