could be a combination of mild slop throughout the system. maybe it's time to jack up all four corners on stands, fire it up and put it in drive.
fwiw, my libby has has a thunk in the rearend its entire life and there is fair bit of slop in the rear axle if you hand-turn the rear shaft. makes a thud you can feel as you let off the throttle. never gotten worse in 10 years. It doesn't make a noise though, just a sensation you can feel.