Dave,
Would it be possile for you to provide a picture of the inside of your overflows?
Sounds like a siphon is being generated and then pulling the water down from the overflow until it is broken when it starts to pull air down through it. The noise could be from when the siphon begins and ends. Just a thought...
Is your skimmer new? If so, maybe the bubbles are coming from the skimmer. If they are, then they should slow down is go away completely after the skimmer has a chance to work for a few days/weeks. You can also try placing a filter or sponge in the sump where the skimmer discarge is going.
Good Luck and Have Fun!!!;
