I have noticed a noise that's hard to describe on my justa 07 with 40k mi. In 5th or 6th gear and speed over 60 mph I can hear a on and off hum. I cannot feel any change in speed or other effect on the car. Any ideas. Most of my driving is city under 50 mph. Thanks

 

It's a long shot but try checking your tire pressure. I had the same porblem when my tires were under inflated.

 

I'll give the air pressure check.