Weird radio "problem"

The other day I when I was handed off to a center frequency I got a
continuous "rx", like a stuck mic, on my Garmin 430. I asked about changing
freq, he said he did not have another available. I explained the stuck mic,
and he queried other pilots on the freq, who all reported 5x5. I started
playing with my stack, and discovered that if I turned radio 2 (an old
King - probably original 1979 equipment) off 121.5, the interference went
away. The weird thing is that comm 2 was turned off on the audio panel
(Garmin 340). Never happened before or after - only on this center
frequency. What might have caused this?

Michael