So happens that my 04 XB12S is doing the same thing, whining sound on no throttle decel. Checked primary chain and it is spec, primary fluid is clear. No visible break wear. Replace front wheel bearings last year. Rear seem fine upon inspection. Had the belt tensioner off over the winter and had not indication of bearing problems there either. Didn't make the noise until this spring. At a loss what it might be. Almost sounds like the SX and RX decel.