What's that loud thud behind driver in lower corner of cab.

[Gregski]

seems like any time I go over some speed bumps a bit faster than I should I hear a loud thud behind me, behind my left elbow, sounds like somebody left a large hammer behind the bench seat (yes I checked, there is no hammer behind the bench seat, lol)

my truck bed is empty, cab is clean, I wiggle the driver side door and don't hear anything loose in there, it sort of seems like it may be under the cab right behind the cab, I honestly don't know I looked under the truck nothing seems lose

any ideas fellers?


1974 GMC 1500 350 manual 2WD