It doesn't really sound like a bearing issue to me; however you may just want to overhaul them anyway. Typically you would gauge bearing issues by turning the axle by hand to easily feel imperfections.
My wild guess about the noise is that it's coming from the cassette 'catching up' with the momentum of the rim, which spins faster initially. As the cassette catches up, a pawl may click into the next position making this click. This would be normal and of no concern.