If it's on the right side, check first the panels in the a/c system behind the glove box.

If its loud and coming from more on the left side especially when the engine is cold, check your airbox (this was the case with mine before, sira na kasi yung mga bolt ng airbox).

The Frontier dash isnt that prone to rattles. I already have 63K on mine and i still dont have any rattles.