Daft as it may sound, check your wheel arch liner...
I once had a strange zuzzing noise on the front at speed (i kniw you cant hear owt, but volvos are very insulated) but nothing to show, that was until I noticed some unusual abrasion on the liner... It had become detached from the bumper, at rest it sat where it should, at speed the air pushed it onto the wheel! Couple of self-tappers it was all sorted.
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