Originally Posted by Duc_Z09

That's a little more worrying then. Are you noticing any other vibrations besides the knocking/flapping noise? It's gotta be something loose that a specific harmonic at 5k is setting in motion.

Was the intake manifold cover (plastic thing on top of the engine) removed and put back recently? That would do it, if it was loose... And it's probably insulated enough from wind that speed wouldn't matter, but rpm definitely would because it's bolted directly to the engine.