Trying to recall a story involving two observers in space stations who kill an alien, conspiring via music

I read this around 2005ish, in English, but I think it was in an anthology of older stories. Humanity is at war with aliens (or maybe just a rogue branch of humanity) Two men are in space stations at the edge of space, looking for signs of invasion. They have a playful rivalry involving trying to stump each other regarding music. An alien sneaks up on them and has them in some sort of state that prevents observation. There's a technique that can be used to escape, but it takes both stations to act in synchronicity. The protagonist knows their communication is monitored, but finds a way to communicate position and timing via song titles.

Answer

This is probably "Tin Ear" by Spider Robinson. It's in his "Antinomy" collection, 1980, Dell books.

... the Enemy’s drones keep popping out at irregular intervals, robot-destroyer planetoids … somebody has to mount guard over every known ’Hole, to sound the alarm when a drone comes through, and hopefully to neutralize it (before it neutralizes us)

...

It was well that Walter and I inhabited separate Pods. We didn’t get along at all. The only things we had in common were (a) an abiding hatred for the government which had drafted us into this sillyass suicidal employ (“. . . before we had a chance to volunteer like gentlemen,” we always added) and (b) a deep enjoyment of music.

...

Both types of Enemy planetoid have only the two tractor beams—but the relative positions of them are one of the chief distinguishing features from the outside. If this was a Mark I, we could both throw full power to our drives— and while they wouldn’t be sufficient to peel us loose, their energies should cross, like surgical paired-lasers, at the center of the planetoid, burning out its volitional hardware. If this were Mark the Second, the same maneuver would have our drives cross in the heart of the power-plant and distribute the component atoms of all three of us across an enormous spherical volume of space. But how could we compute our positions blind, on a sphere with no agreedupon poles or meridians anyhow, and communicate them to each other’s computers without tripping the damned planetoid’s squelch-program? The cagey son of a bitch had cracked Cipher B too easily—apparently it was programmed to jam anything that it computed to be “exchange of meaningful information” whether it could decipher it or not.

...

forever to establish contact. I scratched a telemetry contact and consulted Abacus Al. “How,” I programmed, “can I communicate meaningful information without communicating meaningful information?”
That’s the kind of question that makes most computers self-destruct, like an audio amplfier with no output connected. But Al is built to return whimsy with whimsy, and his sense of humor is as subtle as my own.

“WRITE A POEM,” he replied, “OR SING A SONG.” I snorted.
“No good,” I punched. “Can’t use words.”
“HUM,” Al printed.

...

They exchange hummed song titles to pinpoint the enemy ship location and then crack a lame pun (a Spider Robinson trademark) about how the enemy ship died listening, just "taking notes".