Weird whistling from the rear derailleur Shimano Deore xt, 10sp. The noise happens only when I change gears for the short period of time when the chain is moving from one sprocket to another. While pedaling on one gear it works well and silent. The chain is not new, but is not worn out, no skipping or jumping etc.

What can be a reason for this?
I've come across this before, but never on my own bike. The real cause was never identified, but I believe stripping, cleaning, and oiling the cage and jockey wheels was the fix. That leads me to suspect that a little sideways pressure during the shift caused something, like a bit of grit, to rub.