I found it once and not again, but there is a way to kill that horrible chime when you plug in the mbp. (It reset itself upon a reboot, but it's the best I could do.) Now I can't find that language. I'm not talking about the boot up chime, but rather that stupid cell phone charge plug in noise.
defaults write com.apple.PowerChime ChimeOnAllHardware -bool false; killall PowerChime
It resets upon boot up but this will do it temporarily. I'd love to find out how to disable this permanently.
EDIT: I am so proud of myself. Here is what a dummy can learn to do for himself. After I found that code for Terminal above, I had to figure out how to make that command run upon login.
Open TextEdit and type
#!/bin/bash defaults write com.apple.PowerChime ChimeOnAllHardware -bool false; killall PowerChime exit 0
... and save it on the desktop as "PCK.txt." (Powerchime killer)
Rename that file to "PCK.ps"
Open up Terminal and enter
to navigate to the desktop, where the PCK file is. Then type
chmod +x PCK.ps
Now you can test the script if you want, hopefully it works and the sound is killed!
Next, under the Terminal drop down menu, there are preferences. Under the Profiles tab there is a Shell "sub tab" on the right. In this tab there is a "When the shell exits" section, under which is a box that you can choose one of three options. I chose "Close if the shell exited cleanly," and now when I double click my "PCK.ps" file, a Terminal window flashes and that stupid powerchime is dead.
The final step is to add PCK.ps to my specific user login items. The login items are located in Preferences / Users and Groups / >user of choice< / Login Items. Then, click the plus sign and browse to PCK.ps and add it. Check the box and now try a reboot!
Since updating to OS High Sierra this no longer works! But a much smarter person than me posted a much better solution: Turning Power Chime Off