I know your pain, I started having my issues after 100k as well.
I recently had the tensioner pulley go bad on me, I ended up replacing all three pulleys for roughly $100-$150 (with labor from a small local shop). It was making that same sound. The stealership tried to tell me it was the AC compressor, but it didn't make the noise till after they had replaced the belt and thus put the tensioner pulley into a different position than it had gotten used to with the old worn out stretched out pulley.
Try turning the AC completely OFF not to hot air, but turn it OFF and see if it goes away. Without the extra strain on the pulley, mine stopped making noise. This is why they tried to convince me it was the compressor.
It sounds to me like a very similar sound that I was having when my tensioner pulley needed replacing anyway.
It could throw the belt and cost you more hassle, so I'd get it checked out sooner than later, which it sounds like you're doing, so I'll shut up now
Hope you get it figured out
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