Well, since no one is offering a BL explanation, I defined it on google.com and decided to post an interesting definition...
"This is commonly used in Japanese language. It's a story of attraction between two males, not always sexual or explicit. Romantic attraction is also considered "boy's love." These stories are mainly geared toward women."
Now, do you see why it's important to answer the true origin of BLBLBL? If you don't, we think the worst!
Edit: lol, I see someone did offer an explanation. I imagine that would be quite humorous to see, if you weren't part of the group and didn't know why people were stopping to make that noise.