just remember that after testing with a battery backup as your power source, if the noise is still there then it points to the hardware.
the next thing i would do would be to test 1 cabinet at a time (still using the battery)seperate the two cabinets and strip 1 of them down to the "lowest working configuration" - which means disconnect all CO lines, disconnect all exp cards, and remove all amph connectors. hook up a different amph tail to stations 1-8 and connect only one "known working" phone. pick up on the intercom and if the noise is still there then it's a bum system. if the noise is not there, then start bulding up the system till you find what is causing the noise.
