After my bike is ridden for a few minutes, a whirring sound begins to develop. It is directly engine or wheel speed related because the whirring gets higher and lower in pitch respectively. In addition, there seems to be a little grinding coming from somewhere near the chain. I can´t figure out whether it is the main bearing in the engine or if it is my bearings in my rear wheel. My chain is a brand new Regina so I know that isn´t the problem. PLEASE HELP!! I miss my Yammie.