What could be the problem if a popping, grinding noise can be heard while under light, not heavy, braking? The noise is directly related to how fast the wheels are rotating.

Friend and I replaced the pads in my GF's Honda and then about a month later the noise started. She has a civic with disc&drum and the sound is loudest from the front left wheel.

 

I need to put new strut bars on it, don't I.

 

My guess is a minor misalignment somewhere in the brake assembly, but since you mentioned that the brakes are not the problem, it could very well be something else (i.e. strut bars, etc.). All in all, if this is getting nowhere, have the car checked by a reputable service shop like Midas.