I would power the system down, reseat the modules in the carrier if you have one (if you don't have one skip this step), then power the system back up. See if that clears the issue. If it does not, you could also try going to x10 or x11 and pressing Feature 00, the left Intercom Button twice, and then #728. This is a reset save all which will clear all the ports.
If the noise persists: See if it is internal as well as external bu calling extension to extension or calling voicemail. If it is clear internally then it could be the line ports took a hit. If you have a single-line set, unplug each line from the system and test them with the set to see if there is static when not connected to the system. If they are clear outside the system you probably need to repair/replace the processor to fix the issue.