Diagnose the sound...Creaking/Groaning from rear

[catalina34]

When I accelerate from a stop and the weight transfers to the rear I get a groaning/creaking that is pretty loud. This only happens during weight transfer and then goes away. However, jumping on/bouncing the rear bumper doesn't reproduce the sound.

I have no f or r sways, 7.5wraps, and stock panhard.

I'm thinking it is the springs, but not quite sure.