My bet is drive shafts out of phase, I forgot but I fixed that first, then did caster shim to improve steering. It is a super simple check/fix (phasing). But Georg will know, he's the expert. Very kind of him to lend some help. Good luck and be sure to report back on what it ends up being.
I had a similar problem problem, but only happened when I was pushing on the gas pedal. Sounded almost like a truck engine braking and came with a rattle/rumble. Castor shims fixed it.