Hi,
I had a similar problem with my Toyota Celica. I had a constant squeeking noise when driving around. I took it a few garages had break pads sanded down/greased etc. Still the problem continued and the garages said they could not find anything wrong. In the end I took it to a Toyota dealer. Turns out that it was a stuck caliper. I had to have a new one fitted which got rid of the noise.
May not be the same but something to look at.