Hi I hope someone can help

Kxtd1232 master cab
2x 8 extn modules
1x pri isdn module

Slave cab
Is using 15 extns

My customer has a 1232 master and slave cab system. All extensions (more noticeable on analogue) have a background whine.
I have changed the master cab and cards but still have the same problem. If I disconnect any cards on the system the noise goes away, as soon as you connect it comes back!
If I disconnect the slave cab it makes no difference.

Can someone please help I am stuck!!!

Jim