Buzzez are a pain. There are many different causes of buzzes.
The filament or bulb in the piano lamp can vibrate just from one particular note.
Check the music desk for loose screws. The fall board too.
The jacket or copper winding on a bass string is loose. To correct this, the string has to be slackened and with a vice grip, needs to be turned by the loop end one revolution and but back on the hitch pin. More common of a problem than one would think. Bet its that one.
Check real good for any debris.
As for the double strike, that's a regulation problem, not a manufacturers defect, so good luck getting that fixed under warranty. Deregulation happens through normal wear and tear and is only in need of a simple readjustment. If the rest of the actions regulation is good, just get that one note done when you have the piano tuned. Don't worry though, its not a big deal. You're piano is not broken.