Welcome 
Have any of your mechanics gone in the car with you to hear the noise? There's not much that could knock under the passenger seat.... my 1st thought was maybe cradle bushings, but not sure that would be constant or affected by braking. Almost would suspect the bearing on that side, or a suspension bushing.
Have any of your mechanics gone in the car with you to hear the noise? There's not much that could knock under the passenger seat.... my 1st thought was maybe cradle bushings, but not sure that would be constant or affected by braking. Almost would suspect the bearing on that side, or a suspension bushing.