the heat shield underneath needs to be inspected while running .
should that not be the noise, consider the harmonic balancer. it can have an intermittant rattle at times if bad.
you may call it the crank shaft pulley; it has a rubber insulator inside a metal sandwich which can deteriorate . if very bad, you will observe a pully wobble where the alternator belt turns off of the crank.
its been awhile, but i do not beleive the timing belt needs removal.