I think that was Heartbeat, Bev. I only know that it was written and posted no later than March 1996, when I read it after getting on-line capability and joining FoLCdom.
The scene from ILTY would be an interesting source for the heartbeat idea. It never occurred to me that Clark would be listening for something so much quieter than Lois's breathing, particularly since the director cut from Superman outside the gold repository listening for Lois to Lois breathing noisily inside the vault.
The whole idea of Clark listening to, being comforted by, and recognizing Lois's heartbeat is definitely FoLCLore, but the way Nan has used it in her Home series is just canon. There, she has Clark able to make a good guess at people's emotions by listening to their heartrate and smelling the chemical changes on their skin, which is an extrapolation of what we saw him do in P,ML and ATAI, plus decades of learning what emotions those physiological responses were attached to.
Does it really matter where the elements came from? I don't think so, not unless you're especially concerned about writing a story strictly in canon. Most of the FoLCLore ideas make good sense in terms of character, too, which is why they made it into FoLCLore in the first place, so why not use them? I used FoLCLore on my very first story (A Shot in the Dark). My author's notes says: I've borrowed the Kryptonian sexual physiology from Jiji's [note: Jennifer Baker's] stories because it seemed like the perfect compromise between Clark's possessing superhuman endurance and being "faster than a speeding bullet."
