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 brings up sources in the Rishonim (Siddur Ri Ben Yakar). The reason being that it is only after the removal of the orlah (foreskin) that the baby is 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 (Hishmat Abraham), 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.