hey all, just discovered this thread and wanted to chime in.
I think i got the cow bell noise as well (i would describe it as "clonking/clunking" from the rear wheels when braking at low speed), and I was able to both capture it on video and recreate it at the dealership.
shop foreman said it's a warped backing plate of the brakes (? i could have heard it wrong), and whatever they did, the noise went away (knock on wood).
hope this provides some insight to those that's still troubleshooting this issue.