Brian,
I have seen that before. It looks like to me when I experience this, the graph looks like it instead of plotting maybe once per minute, it plots lets say 60 times per minute. I never associated noise with this when I brought up the noise graph, so I knew on my temp probes were not causing issue. To get out of this mode, I would just switch out of 1hr mode, let it run for a minute or two and then back to auto and the issue was usually gone. I have only experienced this when I was using the 1hr time base of the graph.