I have a 2010 Mazda 6 i 2.5L with 139k miles on it. For about a year now I've been having problems with a squealing and rattling sound from underneath the hood. I hear a tinny, rattling sound when I accelerate at low speeds, and when driving the belt squeals sometimes - it likes to do this right around 2,250 RPMs or when I really mash the gas pedal, like when speeding up on an on-ramp. So far I've replaced the serpentine belt, the idler pulley, the belt tensioner, the upper motor mount, and the lower rear motor mount, to no avail. All the heat shields appear to be firmly in place. Oil is fine and fresh. I'm not sure what to do next - is it possible that the tensioner I put on was a dud/I installed it poorly? I've been fighting this stupid noise for a while now and didn't want to have to take it into a shop and pay, but I don't know what else to check for. Thanks for any suggestions!