I think I may have found the reason for the rattling at low RPM's...I found the message below posted on the internet. What do ya'll think? Does this seem to make sense and do any of you think it cause to be concerned?
Message out from Ford:
Some 2016 GT350 Mustangs might exhibit Piston Rattle from Idle to 3000 RPM
Some 2016 Mustang GT350 equipped with the 5.2L engine may exhibit a rattle or rapping type sound during cold operation at light loads and speeds from idle-3000 rpm. The noise will be most noticeable during light acceleration in the 1500-3000 RPM range and may also be noticeable in the 800-1200 RPM range with neutral throttle blips. This sound is caused by the piston motion in the cylinder bore. This is an inherent characteristic of the forged aluminum pistons utilized in this high performance engine. Once the engine reaches operating temperature the noise will diminish, although it may still be noticeable even when fully warm. There are no durability concerns with this sound.