You could have used a 'cheater
plug' as a quick check without physically modifying anything, now you really need to not use the BTR-800 until you can replace the
power cord.
It sounds like a
ground loop issue. Can you use an extension
cord to try powering both the BTR-800 and the Clear-Comm master station or
power supply off the same
circuit if not the same
receptacle?
You might also want to check to see if Pin 1 of the wired
Clear-Com lines is tied to the shell of any of the connectors on the cables or any related wall plates, if it is then break that connection.
If that does not resolve the problem you might want to consider
Clear-Com: Partyline, Digital Matrix, IP and Wireless Intercoms, it is intended specifically to isolate two connected systems but apparently costs around $189.