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.