I had to do a slight bit of research and I found out that it might be the torque converter. The clutch can generate vibrations for a brief moment while changing gears.
I'll link you to the page that I found as it's a bit wordy.
http://uberlame.com/a6_tsb/Transmission/Chirping,%20hooting,%20squeaking%20noise%20when%20shifting%20between%203-4.pdf
Glen!