Check your tyre treads for... Sorry couldn't resist that...
Ok normally a bearing noise will sound worse at speed or under load. This is why mine started when I went round right hand bends, because the load was on the left side of the car.
Now what may sound like a stone in a tyre to you, may sound like a flat spotted tyre to me, so I would recommend doing the following. This will give you a little more indication if it is going.
Jack your car up at the rear on the side you suspect is making the noise. Make sure that you have your car engaged in 1st gear and the handbrake off. Leaving the wheel in place, make sure your brake pads are free off the brake disc as possible, and then spin the wheel by hand.
If your wheel bearing is going then apart from a metallic noise which will probably be the brake pads catching the disc now and then (which they most likely will), you will hear a dull rumble noise.
If you are not sure, check the opposite rear wheel and listen for the same noise. If the rumble noise is not on that wheel, that suggests the side with the rumble has the bearing going.
If you are still not sure, get it checked by someone who has had a bearing go and 'knows the noise' or get a mechanic/garage to look at. If only for peace of mind.