There is a continuous noise (not a squeel) coming from rear wheels.

I have a 2004 Holden Zafira. There is a continuous noise (not a squeel) coming from rear wheels. This has only happened over the past week. There is no grinding noise. Interesting enough, when I drive slow and pull the handbrake slightly it goes away. Also if I hit the brakes hard when I stop (when driving slow) the noise doesn't return until I use the brakes again in the normal way (not hard).