Start to notice rattle noise from engine bay recently, particularly when temperature around 32F. Not as noticeable in drive, but if put car in park, and rev engine to 2200rpm, the sound is quite loud like lose part is going to break off. Also the resonant frequency changes with fuel level. With full tank of fuel, it's 2200 rpm, half tank or lower, it's around 1800rpm. I guess it's not from the engine itself, but could be heat shield or radiator or belt. Difficult to check heat shield as under-body is covered. Anyone experiencing similar issues?