What can cause a squealing in the car if it's not the brakes?

Originally, squealing occurred inconsistently in the car. Thought it was the rear brake shoes - changed those around January. (Prior to this, I changed the rear drums, rear brake shoes, the front rotors, and front brake pads about a year ago during the summer.) After replacing the rear brake shoes in January 2012, the sound desimated but came back after a few weeks. Rode it out for a few months (tired of paying for brakes that don't need changed). Within the month (April 2012) - I decided to change the front brake pads (the squealing is annoying now). When I replaced the front brake pads, the current pads had plenty of "meat" left on them but figured I would change them - hopefully the squealing would go away. Well - it's been a week with the new front brake pads. The squealing went away for a few days but has since came back in vengence. Yesterday while traveling in the city, the squealing was loud and all the time while driving slowly. Today I took good mental notes of the squealing. The squealing occurs when the car is coasting under 30 mph and the brake is not engaged. When I engage the brake, the squeal goes away. I stop at red lights, accelerate (the squeal starts) and once over approximately 30 mph, the squeal subsides. As well as hearing the squealing, I have also heard a "popping" which sounds like it's under my driver's side seat on occasions. (It's like a rock is being thrown at the car - quick pop). I'm not sure what else to do because I've been told "my car is 7 years old, with 174,000 mi. - It'll make noises". Please help!