Question

Which is the correct possessive form of the humming of the engines of the two cars?

Answer 1

http://grammar.ccc.commnet.edu/grammar/possessives.htm

Go down to the "Many writers consider it bad form to use apostrophe..." in a box part and read it. It talks about the use of posessives and objects.

Answer 2

And if that does not do it, then read the whole thing to see where that falls in the overall part.