If you're not sure if the whine is from the bearings or electrical, maybe you can disconnect a wire somewhere (choose carefully to be sure not to create more problems for yourself) to electrically isolate the alternator, with the belt, etc. still on. This will be a tougher test since you say it is intermittent.

I feel your pain with 5 BMWs.

I do all DIY to save money, plus I like to learn some new skills.

545i: Voltage regulator is dead causing an overcharge after 10 miles of driving. Car sits mostly until spring so I can replace the 37.00 VR. Dealer wants almost 2k and indy is about 1200 or so.

330xi: Alternator died last week. I think the bearings are going out on it as well. Does not charge and is starting to vibrate. I will fix that in the spring for the cost of the repair kit from that ebay seller I mentioned. About 50.00 or so.

The two silver e53s have door carrriers and or window regulators failing.

Yeah I told you my friend :You are a brave soul taking on two E53 X5's at one time. I love my X5 but one is enough for me. Thank God for the E39M5 and E24 635csi solid built platforms.

We have two x5s 'his and hers' 2001, before we bought them it was only because I can fix almost anything; my brush and slip ring kit just came today to fix my alternator. Not a difficult job. I was hoping to replace just the brushes, but wowza the slip rings were shot!

Is the whiny alternator a fresh replacement?

You can isolate if the problem with alternator is electrical by simply disconnecting the output cable; only under actual load will you generate forces that could be audible. If you still have a whine it likely is a bearing; the good news is the bearing kits are not expensive; the bearing + slip ring + brush kit is about $30-40.