Re: Is this a Savannah cat?
Welcome to CF!
No, I do not think she is a Savannah Cat. What she is, though, is PURE LOVELY! What a sweet little girl kitty! She just has a unique whistle-y purr. My BooBoo and Shasta do that, though not as pronounced as your girl. Boo has more of a breathe-y purr while Shasta has more of the whistle. I think it is a unique characteristic, certainly, but I do not feel it is one that could define a breed of cat, especially a Savannah, which has a particular body-type and markings.
I think your friend who told you that probably wanted to fluff-you-up and make you feel better about your kitty by thinking it was rare. ...and in that respect...I agree. Very few cats have a whistle-y purr like yours!