Since a non-combatant is a pretty generic term, I will define it now.
I would like to stay away from choices in the codex, such as Hive Guard, or Tervigons, since they fulfill an active role in a Tyranid army. I will allow Mycetic Spores though, as they cover a more broad spectrum, and aren't really an active participant in an army. The same goes for other types of tyranid structures and flora (though I'd like to stay away from these as much as possible, in order to save them for another terrain contest or something, but I can't stop you).
Things that would be classified as non-combatants are organisms that form a defensive or supportive role in a Hive Fleet. This could be anything from the Teleporter worms used to get from one place to another on a Hive Ship (
), to the Scataphagoid beetles that clean the respiratory tracts of Hive Ships. Other options would include making markers to show which thorax swarm you are using, by modelling little Electroshock grubs, shreddershard beetles or dessicator larvae. You could also make one of the ameoboid creatures that defend digestion pools, and if you want to be eccentric you could create a Narvhal ship for use in BFG. It's up to you.
What I was thinking was to make some sort of Tyranid that formed living tank traps/dragon's teeth.
Is that more clear now?