From the rules:
From reading the Withstand power, it looks like it is meant to be able to resist agg from any source other than the agg from draining. It covers both damage from weapon strikes and exposure damage. However, in the case of taking agg from an iron / normal metal weapon, there is no verbal <type> to remove from the call. Are characters who take agg from iron able to use the withstand power to remove the the iron type from an attack despite this type not being called out loud during the strike?Aggravated Damage from Iron = attacks by normal metal weapons
Withstand: Activate this power to remove the <type> from a damage attack that strikes the caster, or from exposure damage. You must be conscious and able to use self powers to use this power. This power does not work against aggravated damage from draining.