Has anyone else noticed audible interference when using the serial port on the 990 for automation?

I have my 990 connected to a 7700 power amp with balanced connections except for right-surround which is connected with an RCA cable (that balanced output is non-functioning on my 990).

I have a serial cable connected to the 990 from a URC master controller (MSC-400) which is used to automate my entire system, mostly via IR.

When operating the IR repeaters of the MSC-400 or sending serial commands to the 990, I can hear audible clicks on all output channels, regardless of the 990's volume settings. It's loudest on the right surround channel that's not using a balanced connection, but all channels make the noise.

It's been a great pre-amp, but noise isolation like this is irritating. The RCA-connected channel also has a constant hum which is not present on the balanced connections. Very low, but loud enough to bother me if the room is otherwise silent. Easy to take care of by playing some music or a movie of course.