Left-turn snapping noise - Ideas??

[SkinnyG]

'77 Silverado, 350/th350.

There is what sounds like sheet metal being caught in a driveshaft, but only when I turn moderate to hard left turn only. Only there is no sheet metal in the driveshaft.

I ~think~ I can feel it in the steering, but I might be fooling myself.

Usually sounds like it is coming from under the floor, other times it sounds like the front wheel area, and sometimes from the rear axle area. Not at all useful, I know. It also seems to be related (or more easily instigated) by bumps on the road.

Here is what has been done, not all related to chasing the sound - just what has been done, none of which made a difference:

350 fully rebuilt
2500-stall converter added, "tamed" B&M shift kit, fresh fluid
Headers, x-pipe, and dual exhaust installed
Heat shields removed (cat is gone)
Poly engine and trans mounts
Poly cab mounts
Poly suspension bushings (front and rear)
New 1-ton coils
BellTech spindles
Upper a-arm bolts checked for tightness
Lower a-arms moved forward 3/4", cross member rail clearanced
Flip kit, both via bracket and via welded spring perches
New U-bolts
Rear shock extenders
New shocks (front and rear)
Box mounts inspected, bolts replaced
Rear brake drums turned to remove any corrosion on the edges
Front hubs cleaned and bearings re-packed
Fuel tanks removed and truck tested to see if they were contacting anywhere
Control arms trimmed for wheel clearance
Fenders checked for tire clearance
Front brake calipers and dust shields checked for clearance

I cannot find any contact at all anywhere.

Another funny thing, is sometimes it is worse when cruising around 35mph in 2nd, even on the straight.

It is also usually much worse when pulling my utility trailer.

Honestly, I'm not sure where to look, and I'm hoping someone might have some magical experience like this and might know what tree I need to bark up.

Thanks.