That is the sound that recurs constantly in the Transformers movies (and most other big budget films nowadays) when something epic is happening or has just happened.
Optimus Prime just realized that his hero is the villain. BWAA!
Batman must race to save the woman he loves so much he hasn’t noticed that she changed from Katie Holmes to Maggie Gyllenhaal. BWAA!!
I arrive at work right on time and realize that I left my work shirt at home. BWAA!!!
We laughed about that for a long time. But now we have moved on. Now Buddy wants a gong next to his bed. Which sounds like this: GOOOOONGONGONGONG!!!!!