It’s not plagiarism. The songs themselves are obviously completely different. Making an engine noise was one in a pretty standard set of whammy bar tricks that was pretty ubiquitous when guitars with Floyd Rose tremolo systems became popular in the 1980s. So many people discovered this trick independent of each other that nobody can credibly claim to have invented it. It was so common at the time as to be generic and kind of hacky.

In other words, it’s a piece of guitar technique and not an element that can be copyrighted. Which is good because music would become insufferably boring very quickly if musicians weren’t allowed to learn and iterate on each other’s technique.

Well both of us are imitating motorcycles, so it would even be very hard to write down the actual ‘melody’ to compare both. As said by others, it’s also an easy and common thing to do on guitar. Plus both sounds are rythmically a bit different : Motley Crue do not have the fast bit at the beginning of Montrose, they do 3 bends rather than 4 and they are a bit slower. It still may be technically plagiarism but it would be on a very small scale

Maybe. But as far as guitar techniques go, it’s very simple, and when I first started out with electric guitar I discovered it on my own and thought it sounded like an engine, as well as I’m sure thousands of others did too.