Possibilities: Fault in the 48 volt power supply.
Interference from an outside source getting in to microphones.
You need to be more specific about the fault. Is it with no microphones connected?
What wiring is connected to the mike inputs?
Is your grounding done correctly?
I tested the P48 line with a Multimeter, was 47,9v.
The Powersupply is a one year old MPower from Malcom Toft.
The Display on the PSU shows 48v.
I teste two different Power Cables, even a 1m Cable, no difference.
It's not a ground or 50 Hz hum, its a high buzzing noise.
There are two 25m Mogami Looms connected to the Mic Inputs, all with separate Grounds.
I have a Lemo Patchbay for the Mic Lines/ChannelMic Ins, because its a way better connection than the usual (and 40+ years old) Bantam Patchbay.
The normalisation works with bridging Connectors. I removed all of the Connectors, so not mic lines were connected at all,
No difference.
The Noise is there even if nothing is connected to the Mic Inputs...
We also switched of every Light in the Studio, absolutely no difference.
No Rack gear was switched on or was connected to the Console.
I have no clue right now...
Thanks!