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Might also be the backlight/inverter getting ready to go.
A lot of laptops will run the screen at reduced brightness when on battery power. There is a long fluorescent tube that provides the backlight.
I had to replace one not too long ago. Not because of squealing, it went dark.
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At this point I really think it is the inverter.
The OP at this link describes my symptoms almost exactly:
Link
Here is what I think is the correct part, for ~$40.
Link
I am seriously considering ordering this part next day air and then either putting it in myself or having a friend do it. I do not want to wait 2-3 weeks to get my laptop back by sending it to Dell, even though it is still under warranty. They may actually replace more than needs to be replaced anyway.
Thoughts?