1997 Toyota RAV4

Driving at 25 mph or more while my foot is steady on gas, my '97 RAV4 AT runs alright. But once I release it, I can hear a low rattling sound. I am not sure where is it coming from. Stepping on gas again will make the noise go away. Do you have any idea what causes this?
Another thing, I don't know if this is related to the noise I've just mentioned. Sometimes when I step on the brake, even just lightly, it'll create a loud rattling sound. I would say it's a metal hitting another metal. Again, do you have any idea what causes this? I would greatly appreciate any advice/ enlightenment you may have.
Thank you and more power.