The unexpected noise of animationCurve when making it with a script?

Hi, I'm trying to make my own animation using a script. I followed the script reference in unity documentations, but the result is confusing. here's some of my codes.

 for(int i = 0; i < count; i++) {
 ...
 posXKeys[j - startIndex][i] = new Keyframe(time, bones[j].localPosition.x);
 }
 ...
 ...
 AnimationCurve curve = new AnimationCurve();
 curve.keys = posXKeys[j - startIndex];
 clip.SetCurve(relativePath, typeof(Transform), "localPosition.x", curve);

and when the scipt has executed, the result animation looks like this:

the keyframes are correct. BUT there're some annoying noise between these keyframes. They are supposed to be straight.

Does ANYONE have ANY idea what's going on? How to fix this? It's driving me crazy...

Also, these keyframes' both tangents is 0, and when I tried to edit the animationCurve on animation window, even a slight change can make these noises desappear...