Are you trying to locate the setting manually or by using programming software? I remember seeing that ARC-XT has a setting to adjust end codes under SYSTEM>TRUNK SYSTEM PARAMETERS, but I can't recall if that's just for moto, P25, or both. I've been having the same issue with moto type II system end codes and that annoying "ker-chunk" sound after transmissions. Still have no clue how to fix it even after adjusting settings in the software.
The end code setting will not work if you're trying to do an adjustment on a P25 standard system. If it's a motorola system go to Menu>Program System>scroll to system you want to adjust>Hit enter>Edit Sys option>Set end code>select your desired setting>hit enter.
