The script isn't especially helpful. I've edited out the dialogue.
In the background, a strange noise... it begins low... more like a
million voices than a million machines... It gets louder... The sound
continues to grow... and seems to surround the ship... they react...
almost deafening now... The sound swirling inside and all around...
ends with a slight POP. Q APPEARS, naked, suspended above everyone, as
though held by an invisible puppet.
In context, it would appear to be the sound a Q makes when you forcibly remove their powers, turn them into a human, strip them buck naked and shove them headfirst through a space hole.