I had a rattle in my soundbar that went away as I added presssure over the driver side speaker cover. It ended up being a loose screw that actually held the speaker in place. It was much more pronounced when the music was loud. About drove me insane until I sorted it out.
I just picked up an RCI off-road rack with bike mounts, partially for reasoning, I am local to them. It’s a solid setup and reasonable prices.
https://rcimetalworks.com/product/bed-rack-bike-mount/
Xtrusion off-road also had side mount racks available but I can’t find them anymore.