I know this sounds crazy, but I had the exact same chirping noise in my E430.

One day I put the rear seat headrests (all three) in the up position, and noticed that the chirping disappeared completely.

Upon examination, when the headrests are in the down position, they bounce up and down over bumps, and make a squeaking noise.

I don't like keeping headrests in the up position because it blocks my rear view, so I found that when I angle the head rest when in the down position, it stops them from bouncing.

__________________
Paul S.

2001 E430, Bourdeaux Red, Oyster interior.
79,200 miles.

1973 280SE 4.5, 170,000 miles. 568 Signal Red, Black MB Tex. "The Red Baron".