1978 Fiat Spider - Loud 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.

Sounds like either:

1. The donut ring is ****ed, check underneath, just behind the gearbox for the rubber ring make sure it goes all the way around. - about 15 quid plus labour

2. The rear differential has shed a few teeth and they are causing serious doggie doo. - about 300 quid plus labour - this is the most likely.

3. The gearbox is starting to commit hari kiri - about 700 quid plus labour - although it should not give out much noise when coasting. So somewhere along the drivetrain from center to rear.

This is in order of cheapest to most expensive to fix.

Thanks. I've gotten some replies on other forums and every one thinks its something different. Tons of fun.
Seeing as how you actually own the same kind of car I'm putting your ideas on the top of the list. I appreciate it.

Well frankly if the gearbox isnt engaged, and you are costing with the clutch down and the noise still happens it has to be something between the donut ring and the rear axle.

The other thing Ive noticed, from my own Spider experience, is that the discs on mine seem to warp ( and even crack ) making the break pads clatter regularly. So my tip is:

If you coast, with the clutch pedal down, its still doing it, then its either front or rear running gear. If you touch the brakes and it stops completely, its a disc/pad problem. If you turn the wheel and it creaks or makes cracking noises its a front running gear problem ( or worse - mountings ), if the brakes make no difference nor does turning the wheel, its donut ring, rear diff, or rear axle.
In any case, seek out Fancy Engineering, Malcolm Fancy is thorough, responsible and above all highly competent.
Worth driving to Dorset for!

I'm afraid it probably isn't worth driving from California to Dorset.
Thanks for all your help. The previous owner of the car recommended a lil shop out here called Ital Automotives that specializes in older Italian sports cars. I'm a bit afraid of the prices a place like that might charge though.

Are any of these places of any help?
http://p204.ezboard.com/ffiatcenterf...opicID=6.topic