I have a customer with a Unisyn. All the sudden after 15 years he is getting a faint clicking/buzzing noise on all 4 system phones. No new electronics installed close by. I have a couple dozen of these out there & they have always been a sound running system. It is a no frills system but by and large solid as a rock.
I have moved the stations to the un used ports and the noise is still there. any thoughts!

Delete this posting.
The problem is the KSU.