What the hell is that noise and how do I get rid of it?

I'm pretty sure I know what you're talking about. I experienced this myself as well. It's the sound of Reave's sound effect looping over and over. I think this is a bug, as I have not yet figured out a consistent way to stop it. You can see this person cast Reave here in singleplayer, and the sound effect lasts for a good 20 seconds.

I've had it continue to loop for me even after the wave was finished and all the enemies were dead. There doesn't seem to be anything you can do about it. :/ It's even more annoying than running effect soundlooping in the lobby menu that I've had occur.

The only way I can guarantee avoiding it is not teaming with any Drell Adepts.