mines had a 'rattle' or 'whine' of some sorts for 3-4 yrs now.
i've always thought it's backlash or worn bearings in the 'box?
when replacing the aux belts i spun the alternator and AC by hand, nothing too loud to suggest bearings on those

loud stereo is a good call .....new HU, new component door speakers along with tweeters in pods alongside 2 other smaller ones and a big black box behind the passenger seat sorted it........