so, maybe this is a factor that you can drop when comparison to other summers, BUT that metal is probably generating heat and retaining it and I am sure that you are hearing that metal contract and expand which should make a cracking or poping sound.
Is your attic area ventilated? If not, it needs to be. Then make sure that your insulation in the attic is adequate for the home environment. Remember, every degree in temp that you lower the attic, the inside will be cooler by 4 to 5 degrees.