On FreeSpeak 1.5.7 if you assign one of the Party Line interfaces to a group, and then create a "CALL" {route audio} to that group from the other PL interface, and assign a talk and listen, or dual talk and listen, or talk and forced listen activation, the base station can go into motor boating. The symptom sometimes appears only after Auto Nulling the Party Line interfaces.
How to check for the problem:
Open the Configuration Editor and load the current configuration. Go to the Ports tab and click through the ports until you have PLCHA displayed. Make a note of what that call assignment is. Do the same for PLCHB.
If either of both of those Call Assignments are to a group go to the Groups Tab and examine the membership of all the groups paying special attention to the ones you noted in the step above.
If you find either PLCHA or PLCHB listed as a member of a group then you must do this:
Go back to the ports tab, Go back to PLCHA and turn the activation (the window to the right of Call Assignment) and in the pull down menu select “Talk” ONLY. Do the same for PLCHB. Check “Reset On Apply” and send the changes to the base station by clicking on “Save & Apply”.
Once the base station has finished resetting itself check to see if the noise is gone and test for two way communications with both Party Lines.

Solution update: FreeSpeak v2.0 introduces a new "WPL" Wireless Party Line Line feature, which was designed to fix this issue. Please contract your local Clear-Com dealer for more information.