It's basically same as ～んです。It adds some emotion to the statement, and make the conversation flow. Especially, it is used to give the background of following statement or to explain something. In this case, I think the speaker expects the lister to ask "What's that?"

The んだ ending is used when talking under some premise. And it can be ANY premise. In this case it could be the reason for something. It might also be that the speaker considers what he is saying as something important to know. Like I said, when speaking under ANY premise you use the ending んだ. Tricky, huh?

There are sentence variations such as... びっくりすることが - ある (presentence) - あります (polite) - あった (pastence) - あるのだ ( "profession, order, decision or emphasis" of verb) in traditional way - あるんだ ( "profession, order, decision or emphasis" of verb) in frendly way - あるのか( "profession, order, decision or emphasis" of verb) in questionary sentence - あるんだな( "profession, order, decision or emphasis" of verb) same as "あるんだ" but spoken in a particular dialect. .... etc I know its kind of complecated; however, in most of the time you hear "のだ", or "んだ", it is mentioning to put an emphasis on the verb of the sentence that I believe. Though its in Japanese, I hope you could also refer the link below. http://www.lang.nagoya-u.ac.jp/nichigen/menu7_folder/symposium/pdf/7/12.pdf