2WD or 4WD?
How many miles on it?
Could be an outer CV joint if you have a 4x4. (hence the grease leaked from the boot)
Could be worn out brake pads. Sometimes they are loudest in turns.
Possible wheel bearing. Does it sound like an airplane trying to take off and get worse with speed? Does it get louder during lane changes in one direction or the other? (or is it only low speed left turns)