I'm wondering if you have slack elsewhere and the reduced movement from the firmer bar is making that slack move differently than it did when there was more side-to-side flex. It might not be actual contact between components (bar to control arm, etc.), but something like slack in the control arm bushings abruptly being forced to articulate slightly differently than how they've been worn in.
What if you roll about 5 MPH and stomp the brake, in forward and/or reverse? Try this with the wheels straight and with them turned in each direction.