Phone LIne Noise – PABX (NEC SL1000)

Hi guys. I got some noise on My PABX system and can’t figure out what is causing the problem.

I got 4 CO Line & 22 Extension Line In NEC PBX SL1000 right now. Problem is Only Income/Outgoing call from CO lines are having noise and extension lines are fine.

Things I alrdy check and confirm are below :
– All CO Line are clear before entering PABX
– I had Alrdy switch CO port from original blade 1 to blade 2.

Please help me guys.