My customer has a DSX-80 with 5 analog trunks and 11 digital stations. Recently calling customers started hearing "howling" after the call transferred from the AA to a station. The noise sounds a little like modem noise but more irregular and with sort a static. At first it seemed to happen only occasionally, but now happens often. Only happens on incoming calls. The caller hears the noise but the station reached does not.

I replaced the CO card and the problem continued. Finally I monitored a test call on a butt set on the CO side of the system. Once we heard the noise, I pulled the bridging clips to the system and the noise stopped immediately. So it's coming from the system. Bad CPU card or any other ideas?

I'm forgetting more and more on this system so this is a guess.
Sounds like the internal modem is answering, make sure its disabled in program 1102

What dans said or you have something else bridged on that line like an alarm. I would check that cabling CLOSELY. A LOT of alarm guys snip the block wires and hide their splices behind the blocks.

You nailed it dans. Thanks so much! Now it seems pretty obvious, but it nearly always does when you get it figured out.

Coral Tech, thanks for you input as well. I did check that and found that the alarm company ahd changed lines to one in the hunt group. I jumpered around the alarm and found that the noise went away when the system clips were pulled, pointing to the system itself.

Premature celebration. Customer called again and said the noise continues. Now I'm thinking digital station card.

Did a little more trouibleshooting after a tip by another tech. This system has one line that DIDs to a station, so doesn't get picked up the AA. I did 10 inward test calls to it and heard no noise like what we're hearing on the 4 hunting trunks. Does this point to the intramail card as the souirce of the noise after AA transfer?