So I was driving my 1980 300TD along the highway when suddenly I heard a loud rattling noise. I immediately pulled to the shoulder and called a tow truck. Before it arrived though I did some testing to see what I could find out.

1. The noise seems to come from directly underneath and between the two front seats (trans or drive shaft maybe)
2. It makes no noise in first or second gear, not until third gear and then louder the faster you go.
3. Shifting and speed do not seem negatively affected. I only noticed loud noise.
4. It's a distinct rattling noise, not a "grinding"

Any thoughts or impressions? Thanks.