An old or weak battery cannot filter out ripple from the alternator as effectively as a good battery and can cause alternator whine. You can try running a return ground lead from the amp's gnd to the radio gnd and see if that helps with the noise. What type of interconnects are you using? Are the mounting screws on the amps going through the vehicle's body? If so, this can cause a ground loop and introduce whine into the system also. Use a piece of MDF underneath the carpet mounted to the floor, then mount the amp to the MDF to eliminate the possiblility of a ground loop there.
The Infinity amps are hit and miss with alternator whine problems. Of all the amps I install, the only time I ever have a noise problem to diagnose is when I'm installing the 4 and 5ch Infinity Reference amps. You have to cross your t's and dot your i's when installing these f'ing Infinity amps.
