Question on speaker "Pop" ?

[Tall Dog]

Not exactly sure if this is a receiver issue, or a speaker issue, or a blu-ray issue, but here goes.

Every now and then when we are watching a blu-ray movie, I will get a loud "pop", from my surround speakers. It usually only does it one time somewhere during the movie. It almost seems like a "build-up" of electricity that "pops" out, and then it's ok after that.

I have a Yamaha 3800 receiver and a Panny DMP30 connected to a 7.1 speaker setup, plus two more "presence" speakers. I also have a Velodyne sub.

All my speakers are matched, Yamaha 6-ohm except for my two rear speakers which are Bose. I have the receiver set to 6-ohm.

Anyone else experience this, or know what might be causing it?

Thanks