Squeak on 1995 BMW 740i

I have an annoying squeak (almost bird-like chirping) coming the the back seat area of my '95 740i. I've looked around but haven't been able to locate the source of the squeak. Any thoughts?

2 answers
I have same car that had the same problem when I was idled at a stop light,etc...It turned out that a small bolt to the muffler hanger that secured the hanger against the chasis somehow vibrated loose. The problem was cured after the bolt was tightened/replaced--hope this helps :)
May be the brake pad wear sensor beginning to hit your brake rotor, some brake pads come with a thin metal tab that hits the rotor and make a chirping sound warning you that your brake pads need replacement