Strange noise from back of 2000 Nissan Frontier?

Occasionally I hear a noise during low speed driving in my Frontier. It's hard to describe it or pinpoint where it's coming from, but I think it's at the rear of the truck. It doesn't sound much like a squeaky brake but when I apply the brakes the sound fades away. Is this a wheel bearing or axle issue or it just the brakes?? I know it has drum brakes in the rear.