You can check my web site, and look under Noise ID. I suspect it is ignition noise, but it also could be injectors.
Grounding the mount isn't going to help much to improve the lack of ground plane. The only way to fix that is to permanently mount the antenna which requires holes.
Questions are, have you bonding anything on the vehicle? Have you used any beads? How is the radio wired? Is it remoted? there's a few more, but after you look around the web site you'll understand.