1978 Fiat Spider Making Weird Noises

Hi, I just got a 1978 fiat spider about 3 weeks ago and its great, but I just had my first major problem. I was leaving a parking lot in first gear and I heard a loud WHAM! It sounded like two pieces of metal slamming together right underneath my car in the center. I continued for a minute with problems, but after a redlight when I accelerated again it made the noise again, 3 times. After this I pulled over and peeked under my car and didn't spot anything immediately wrong (for example a zombie dragging under my car ). I drove the rest of the way home very slowly and didn't hear any more of the loud slams, but whenever I was moving and went into neutral and coasted I heard lots of little clangs (about 5 clangs a second the entire time I coasted). Any idea what might have caused this sudden problem and how to go about fixing it. Also, if it something I could easily fix, how much would a mechanic charge? Thanks for taking the time to read my post.