1st thought is...dust cover. It's prolly rubbing on the flywheel at times. I would remove it, and see if the noise is still there.
Not really anything inside the trans that would make a "squealing" noise like that. You say it only does it in drive, but you dint say if that meant while moving, or sitting still.
Another thought is that it could be the engine oil pump. Hot, in gear, at idle, pump speed is rather slow....it could be making a noise like that.
If removing the dust cover changes nothing, the only way to determine if the noise is trans or engine would be to remove the 3 bolts holding the torque converter to the flywheel, push the converter back(toward the trans), and start the engine. At this point, the trans is not in the equation at all. Only problem with this "test" is that yous ay it need to be in drive. With the converter disconnected, putting it in drive wouldn't load the engine at all, or pull the rpms down. So...I would lower the engine idle rpm and see if the noise was still there.
Or.....just turn the dang stereo up a notch or ten....
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