The person you see and hear working amongst the crowd of bidders is known as a ringman. This individual is part of the auction team and is an extension of the auctioneer. The job of the ringman is to convey bids back to the auctioneer from the crowd.
When bids are received in the crowd, the ringman will yelp to signal the auctioneer that they have received a bid and to increase the bid amount. These individuals are also there to help answer questions you may have while the auction is being conducted.