This may not be relevant, but a guy I work with had a dodge grand caravan that similar symptoms, but felt through the floor. It was aggravated by turning, especially to the right. He had a mechanic troubleshoot and said all kinds of possibilities. I drove it the next day and noticed it occurred in neutral rolling through a corner, but disappeared as I varied the RPMs. Bottom line: a quick google search turned up a TSB that required replacing the power steering hose from the pump to the rack. $80 and fine now.
I doubt this applies to bonnevilles, but I never would have guessed a power steering hose could make that much racket.