Strange Noise Coming From Vivo Tab RT

My mother-in-law and sister-in-law both purchased a Vivo Tab RT from their AT&T store within days of each other. Both have developed this random, buzzing sound coming from inside them. I did a quick search and didn't see any substantial articles on this. Has anyone else encountered this problem?

Thanks!
Brian

Is the buzzing noise constant or does it seem to pop up only once in a while? I would contact the AT&T store as the units might be defective. I am not sure what inside could cause a buzzing noise, unless it is the speaker.