"Bud bud" is the sound Indians (brown ones not red ones) make when they talk. My mate reckons they also say "ding ding" afterwards, but I don't see it.
While walking in the hallway, Katie and her bud-bud were reprimanded by their teacher for accidently knocking freshmen students over with their headphone wires.
Christian: Yo I think Nov' and I have smoked so much together that we've become bud brothers. Jeff: Have you mixed your weed with him yet? Christian: Well, yea but isn't that blood brothers? Jeff: Oh yea, that's right. Christian: You silly vermin.