Salôn / Sulûn
Both forms are examples of Tolkien trying to decide on a derivation for the name of the river Lhûn (or Lune). He never seemed to fully settle on what the etymology was, so these words are not truly a part of the "official" Khuzdul corpus, if there is such a thing. However, we can probably go ahead and incorporate the root S-L-N "to fall, descend swiftly". Either word would probably be an active participle, used substantively, such that they mean "a thing that falls or descends swiftly".
It's tempting to see Sulûn as a verbal root S-L-' or S-L-L, and then the same -ûn suffix seen in Nargûn and Tharkûn. The translation would then be the "Falling One". However, since Tolkien here specifically indicates the root is S-L-N, and we have an example, felak, of a noun that performs an action without the -ûn suffix, I think we can discount this idea.
Vinyar Tengwar, Volume 5B, issue 48, pg 24