as for the RS "3 hz"; counting peaks for the 16 second sample I get 2.1 hertz if that is what you are observing.
are you having to physically remove the O2 sensor from one exhuast and move it to the other?
are the locations about the same distance from muffler.
what is your idle rpm
normally the first thing I do when trying to determine the cause for similair "noiz" is to look very closely at instrumentaion to besure I don't chase something that is not there. that would include knowing how the instrument samples and displays