I hear it frequently if other players are around, PvP or Fleet Action or DSE.
I think it's people spamming Full Impulse or maybe Rebalance Shields. Reason I say that, I was paying close attention to when the sounds started and stopped while fighting alongside a bunch of folks in the Borg DSEs. After combat, I'd hear it non-stop until Red Alert went away, then it was replaced by people Full Impulsing to the next group.
So I'm gonna go with "I can hear other people's interface noises sometimes." It's only a lightly-investigated guess, though.