When riding normally, I shift around 4k. Between 4500 and 5000 rpm I get a bit more engine noise but once I hit about 5000 rpm it smooths out considerably. Probably the smoothest operation comes with rpm over 5000. The bike seems to settle in at that point. I ride 70 - 75 on the interstate with no problem. It still has plenty of torque in 5th gear to pass promptly. On back roads I can run for hours at 60-65 mph and it runs very smoothly. The only place I get that extra buzzy noise is between 4500 and 5000 rpm, regardless of gear.
Chris
'81 CB750F