The BRAAAM doesn't play during the opening credits, because there are no opening credits. And when we see "Inception" it's just plain white letters on a black background, at the end of the film. And that sound actually doesn't happen even a single time in the film. BRAAAM!
Above: The reality. Below: BRAAAM.
All those things you remember are from the trailers. The BRAAAM is all over two different Inception trailers (and emulated in many, many, many, many, many trailers since), but it isn't in the film. In fact, there is some controversy over whether or not it was even created by Inception's composer, Hans Zimmer. According to Indiewire, Mike Zarin and his team created the sound by building on the repetitive, percussive motif used in previous trailers, then Zack Hemsey put it into a song that really made it pop. According to Hans Zimmer superfan Hans Zimmer, it was made by Hans Zimmer.
The memory that we all have of that BRAAAM playing during the movie (especially playing during the nonexistent opening credits) is completely false. That means that the trailer for Inception inceptioned us into thinking the Inception sound was in Inception. That is impressively meta. Just like when The Prestige tricked everyone into thinking it was a good movie.