Another thing I noticed before the 02 sensor problem started with my XB12R .
When out for riding for say 20/30mins a whining noise would appear ( speed was around 35mph or 60kph ) .
I am guessing its a bearing somewhere .
When clutch lever is pulled it goes away so that kinda rules our rear wheel or final drive bearings I am guessing .
When starting out on ride there was no noise .
It was quite loud as I heard it while wearing helmet while riding .
I realize it could be a multitude of things but any ideas would be a help .