I posted on here earlier about my 2001 Grand Am GT. I found the ABS problem - a broken wire - now it stops in a straight line but there is some noise and "chattering" of the brakes especially when braking hard, but also at about half pedal in any type of braking. To recap, I replaced all four hubs due to an audible bearing noise when I bought the car. I also replaced all front and rear brake pads and had the front rotors turned. I did not have the rear rotors turned. The old pads were not in bad shape, about half life left and no damage or signs of glazing on any of the four wheels. I also replaced the lower control arm bushings on both sides, as one of them was torn.

I plan to go back and recheck everything - make sure hub and axle bolts are tight, check for play in the wheels/suspension, etc. but I also have a 2003 Grand Am that I bought shortly after buying the 2001, and it seems to do the same thing so I am wondering if this is just normal operation of the ABS system (the 2003 has drum brakes in the rear). I also read on another site that noise and chattering of the brakes is simply the ABS system doing its job. I might note that these symptoms occur on dry roads, I haven't had the opportunity to try it on wet roads or gravel.

What about under light braking?

Sounds like normal operation of the ABS

Was this chattering there before?