This is of course referring to the “Dean Scream” :
Dean attended a post-caucus rally for his volunteers in Iowa to deliver his concession speech, aimed at cheering up those in attendance. Forced to shout over the cheers of his enthusiastic audience, Dean didn’t realize the crowd noise was being filtered out by his unidirectional microphone, leaving only his full-throated exhortations audible to the television viewers. To those at home, it sounded as if he was raising his voice out of sheer emotion. Recordings from within the crowd made it clear that Dean was shouting in order to be heard over the cheers of the crowd but it also showed him to be be highly emotive.
Many in the television audience criticized the speech as loud, peculiar, and unpresidential.  In particular, this quote from the speech was aired repeatedly in the days following the caucus:
“Not only are we going to New Hampshire, Tom Harkin, we’re going to South Carolina and Oklahoma and Arizona and North Dakota and New Mexico, and we’re going to California and Texas and New York And we’re going to South Dakota and Oregon and Washington and Michigan. And then we’re going to Washington, D.C., to take back the White House! Yeah!!!”
This final “yeeeaaaahhh” has become known in American political folklore as either “the Dean Scream” or the “I Have a Scream” speech (satirical of I Have a Dream). Interestingly, while most listeners of the speech considered the scream some version of “yeah!,” many in the print media, such as Time Magazine transcribed it as “yearrgh!” or some variation thereof, giving readers a much different impression of his tone than those who saw the video.