I found a nice list of onomatopoeic words here
http://learnkorean.blogsome.com/2007/05/17/p56/
but I can't find 뚬바뚬바 - anyone want to help out? Here's the context:
난 아카펠라를 했었어요. (지금은 못하고있구요.^^). 뚬바뚬바 ㅋㅋ
I can only assume that the person saying the saying the sentence was just imitating the sound of acapella