Audio problem: When the uptight mom asks how to get to San Francisco the guy at the bar that offers to take her lifts his glass up to take a drink and then we hear the overdub of the ice jangling in the glass. The jangling continues into the next shot after the glass is stationary and no longer near his lips. (01:22:10)
Audio problem: In the restaurant when Mitch says, "We can get Cathy at Annie's now" his lips are out of sync with what he is saying. (01:28:10)
Audio problem: As the cameraman moves in on Mitch and the birds in the kitchen, listen very closely and you can hear his wheels squeaking on the floor. (01:38:00)
Audio problem: As Mitch hammers the mirrored cabinet to the front door they overdub a second hammer sound over the actual sound to make it more intense. If you listen closely they don't get them all right and they miss the very first swing. (01:43:50)
Audio problem: When Mitch turns on Melanie's car radio he turns the tuner knob and stops on the station that he eventually tunes in, goes a little bit forward and then back again to the same spot. The over-dub is not correct as we would have heard the announcer speaking the first time he landed on the station. (01:53:55)
Audio problem: At the end of the movie when they're walking out of the house you hear a door open, but the doorway is empty - it's just an open space, with no door. (01:56:40)
Audio problem: That is not a Martin outboard motor sound when Melanie is on the bay; it is a two-cylinder OMC outboard motor running (in neutral).
Answer: It is never explained, probably because it would ruin the mystery of the film to spell it out.
If you are a Hitchcock fan, you pay attention to dialogue. They mention quite often that it started Melanie's arrival, of which she brought 2 caged birds. The rest is implied.