Speaker pulsing with radio powered off

when I shut off SSDR and then turn off the 6700 there is a pulsing speaker noise I hear on my amplified speakers. It is about 1 pulse per second and almost sounds like a WWV pulse except its around a 400 cyc tone. I hear this with everything disconnected from the 6700 except for the powered speakers and the 12V supply. If I remove the 12v supply and plug it back into the radio the speaker pulses stop. I can duplicate this each time I shut down SSDR and turn the 6700 and switch it off via the front panel power switch. The light stays solid amber at that point. The pulsing is not there during normal operation of the 6700.
My radio ethernet port is connected wirelessly via an ASUS ea-66n ethernet adapter direct to my ASUS RT-N66u router upstairs. (I also unplugged the ethernet and the speaker pulses remained until I pulled the back panel 12v power)
unplugging the powered speakers from the 6700 of course will stop the pulsing :-) until I plug them right back in again.
I searched to find other audio noises at power off reports but didnt see exactly what I am experiencing.