might be the exhaust heat shield that goes over the exhaust manifold. Mine is a bit loose and it starts to "tick" every once inawhile. All you need to do to confirm it is or is not that is to touch it with something to hold it steady.. if noise goes away, that's it. it is on the passenger side of the car too.
Or maybe a leaky exhaust/bolt/exhaust rubbing against one of the heat shields down there?
or could be valve lifter tick