Do presidential pardons need to be announced publicly when they are granted?
They are a matter of public record, so yes, in a manner of speaking.
Presidents can choose (or not) to make public statements regarding their decisions to grant or deny clemency but they have no obligation to do so either way.
Requests for clemency which have been granted are listed publicly on the Office for Pardon Attorney's website along with information about the original conviction etc.
All requests (granted or denied) are public record and subject to Freedom of Information Act reporting requirements. Thus, any interested parties can FOIL the information from the Office of the Pardon Attorney.
https://www.reddit.com/r/Ask_Politics/comments/5axh4m/what_guidelines_or_rules_do_american_presidents/
CNN said some folks are rushing to make freedom of information requests. This whole thing is rotten to the core.