On the April 4 Show, the caller with the Echo mentioned a squeek that the mechanic has been unable to correct inspite of replacing many parts.
I had a similar problem with a Honda Civic. The problem was related to the trim ring. The tabs that secure the trim ring to the wheel were scraping agains the corrosion on the rim rim twice per wheel revolution. Removing the trim rings made it go away. Reinstalling them brough the noise back. Greasing the interface would alleviate it for a while.
I think the Echo has hub caps that would be retained the same way. Remove them and see if the problem is gone.