My car's left rear wheel has squeaky noise for months

What could be the most possible cause? My 2010 Cadillac SRX(FWD) left rear wheel(disc break) has squeaky noise for months. Most of time the noise is quiet, but some times it turns very loud on the road especially in low speed. This happens about 5% of driving time. Really not sure what triggers the noise, maybe a bump or left turn. Once the noise is triggered, the noise hardly stops by any action except one way: Turn off the engine and restart it again. Sent it to dealer to do a tire rotation and install a butyl tape insulator. It became quiet for 3 week and now it's back again. Any idea will be appreciated.