Well, I am currently watching The Lorax with one of my kids and have noticed some strange sounds during dialogue. I have only noticed it with this movie so far.

It sounds like bubbling, distortion, and/or popping. It is only during dialogue and not action/music scenes.

The video is over HDMI and the audio is over coax. I have an Sony ES receiver, before the days of HDMI.

My next step is to switch the coax to optical, and see what happens.

Any Ideas?? Thank You!

1st thing I'd check is if it is repeatable. If you rewind and play the same part over again does the anomaly repeat itself in the exact same way? If so then I start to think it is the source material. If not then move further down the chain as you suggested by switching from coax to optical.