It could be a lot of things, but your use of the word "jingle" reminded me of a problem I ran into with my 67- it sometimes made a "jingle" sound when hitting a bump. As I remember, when I replaced the rear shocks, I discovered a problem with the rear mount where either a washer was missing, or something broke, so that one washer was not pressed against the rubber bushing like it should - it was suspended a little and that allowed it to jingle against other metal when I hit a bump. THe sound drove me nuts for months until I figured it out.
Check this and the suggestions in the other posts. Good luck
1967 Fastback, 289, C-code, AT, PS,
motorsport intake and valve covers, Edelbrock 4V, cam - otherwise stock
dark moss green, parchment interior,
GT rear valance and tips