I have the same phenomenon with my 98 V70 T5M. It emanates from just to the right of the radio. It sounds like a vinyl-on-vinyl squeek which might be caused by car motion, but it persists even when sitting in the driveway. And sometimes it sounds like the knocking noise that you described. It stops when I shut down the Climate Control System. I strongly suspect that it is defective vent duct continuously cycling open and closed.
I am hoping someone will have experienced and solved the same problem. I am resigned to eventually doing an "open and inspect", but I hope someone can save us too much extra work.
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Dave in Virginia; 98 V70 T5(M); 98 V70; 88 245 DL; 67 MGB
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