Apparently, she had some phlegm lodged in her throat, and believed that the longer the "BLEHHH" the more chunks of phlegm she could move upwards into her mouth. It was pretty gross, and as expected my "WTF" face didn't faze her at all. Even the guy inside of the MTR service counter looked disturbed.
It's moments like these when the evil thoughts occur, like how I imagined hacking up and spitting a giant phlegm ball into her face (if only I knew how to hack phlegm up). Unfortunately, it's a skill I never picked up, but the thought of doing so brought a small smile to my face. :)
(And before all you haters post a nasty comment below -- yes, I'm a horrible, horrible person :P)