Why do cats lick car bumpers?

All of my cats (got 4) like to lick bumpers of cars for some reason, and when they turn when I call them they have a mental (but funny) face on them, at first I thought my one cat was licking bugs of off one car or something but then I started to notice he started to make a habit of it with any and all vehicles that park near by (don't worry it's safe roads) and then I noticed that my other cats were doing, but also other stray and neighbours cats, I've discussed it with many other cat owners who haven't noticed it, but the car owners said they haven't washed their cars or anything, so what could it be? I'm honestly co side ring keeping them because it's like its their drug or something