What side of the car is it coming from? and are you sure its coming from the engine and not your chassy because i have the same problem but i leave it as wear and tear.
it goes away when the engine has been warmed up. its coming from the engine....and recently ive been hearing other rattling noises when ever i accelerate.... its coming from the exhaust i think....i wonder if Mazda would fix it with no cost cause i am still under waranty