A popular phrase and bumper sticker from 1970s. It means that people are having sex in the van if it's rocking and they don't want you to come in.
Bumper Sticker: If this van's a-rockin', don't come a-knockin'!
Van: *rocks back and forth*
Steve: I guess I shouldn't be knockin'.
Van: *rocks back and forth*
Steve: I guess I shouldn't be knockin'.
by Dumb_Bass September 26, 2020