Transmission question: whistling noise while shifting

I have a 2004 S10 pickup with ~2900 miles on it. I noticed during the last 200 miles or so a funny noise that accompanies shifting.
I would describe it as a whistling noise, like wind through reeds... that
almost has a ringing sound with it. It seems to happen when the torque converter unlocks and goes away shortly after shifting up. Nothing feels wrong at all. Also, it doesn't happen until everything is warmed up a bit, maybe 10-20 minutes of driving.
I'm going to stop by the dealer and have a service guy drive around with me for a listen, but I thought it would be worth asking here as well.
Thanks, Rand
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Follow-up:
My brother thinks it could be the heat shield on the catylitic converter. I tapped on it a bit and occasionally heard a similar sound. Maybe that's all it is, and shifting gets the rpms to the point where it vibrates just right.

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