Left back wheel making noises when accelerating

[Jessica38]

Can anyone offer advice? Periodically, my back left tire makes a rattling noise, as if a rock is stuck in it or it's going to fall off! After the initial acceleration, it stops. It doesn't happen all the time, never in the morning and usually only in the afternoon or evening after I have been driving awhile. We have taken the tire off and have not seen anything, nothing seems to be loose. This is very puzzling.