For some reason, I have it ingrained in me that a baby boy's name before the bris is a secret and should not be shared. Are there any sources for this, or is it something made up?
I don't know of a halachic reason, but I can think of some good practical reasons. With naming after people being a way to memorialize family members who have passed on, different family members may have different ideas about which family member is most important to remember. You really don't want to go into a Simchah with In-laws fighting with each other and with you over which deceased relative was more worthy of being remembered. The custom that parents must decide the name, combined with the practice of not announcing the name in advance minimizes (although doesn't eliminate) lobbying and arguing and hence bad feelings.
Not announcing the name in advance also leaves the parents' options open to change their mind up to the time of the Bris. I've known parents who went back and forth over which name to choose right up to the time of the Bris. If you announce the name in advance it makes it much harder to change your mind.
This is a good question which wasn't really answered in the last seven years. I found a number of sources
R Moshe Taub (here, part 1) quotes Rav Gedalia Shor, that we wait until a boy’s bris to give him his name, as opposed to a girl who typically receives her name before she is eight days old. Since until the bris the boy is somewhat incomplete, his purpose is also unclear. Once the bris is performed he is then ready for his name
Halachablog writes that the reason is that only after the removal of the orlah (foreskin) is the baby in his perfected state and prepared to receive his Jewish name (Chesed l’Avraham 2:52 from the kabbalistic master R Avraham Azulai)
R Eli Mansour writes the Sephardi custom is to follow the Sod, the opinion of Kabalistic teachings, and refrain from naming the baby until the time of the milah, even in cases the milah is delayed.
Tzitz Eliezer 18:54 brings these and other opinions, tracing what he calls "this holy minhag in Israel" to the times of the Second Temple
See here for more on the special case of a sick baby whose brit mila is delayed - where poskim are split whether to give the name before the brit mila or wait.
Rav Pesach Feinhandler in his Avnei Yashfei 1:196:6 is puzzled by this custom of people not revealing the name of the baby before the bris. He doesn't understand the insistence on not doing so. He explains that if one knows that the bris won’t be done in the correct time then the name should be given before the bris (some say better after the eighth day). He ends off saying: "nevertheless I could not find a reason to be makpid on this like the minhag of the people".
In the sefer Rav Feinhandler printed a letter from Rav Seraiah Deblitzki who wrote to him saying that he thinks the issue of revealing the name before the bris is Ayin harah.
The question shows a basic misconception. Before the bris the baby's name is not a "secret" — it doesn't exist. A boy is usually named at his bris, so before that he has no name. The parents will have discussed what name they will eventually give him, but they haven't given it yet so there's nothing to keep secret.
The same applies to a girl; until she is given a name she has none at all. After all, how could she have one without her parents having given it?
See the language used at naming: ויקרא שמו/ה בישראל. Not "his/her name in Israel is", but "let it be called", or "it shall be called". It takes effect only from that moment.