Unnerving noise from rear wheel
 

Hey guys, I tried searching sheldon brown's website (as well as some threads on BF) for tips about sounds coming from wheels, but I couldn't really get a match to my situation.

I have a set of Shimano WH-R500s on my road bike and the rear wheel makes a slightly-hollow popping noise as it revolves. This happens whether or not there is weight on the wheel (me riding vs bike hanging from rafters with wheels spinning). The sound happens once per revolution.

The wheel is within +/- 2 mm true. I suspect that it has something to do with spoke tension, but I can't be sure. Does anyone with more experience know what might be causing the sound?

Thanks!

PS - I was also wondering if a soft hollow popping noise from the wheels during a sharp bump or pothole (like, say, the 4cm ledge from driveway to garage taken at 15kph (~9mph)) can be ignored or if it is a sign of dangerous stress on the spokes.

Thanks again :)