Phonology question

Okay, a sound has popped up in my new language and I don't know what to
call it.

I'd call it an alveolar fricative, but I already have /s/.

It's like a /t/, except the tongue is relaxed, arched slightly so the very
tip touches the alveolar ridge.  Sounds a bit like a whistle, and it
occurs at the end of words in my new language.

--Patrick