They tell me the flapper is loose and is in intake hitting props. Where is intake that has props?

What? Is this on a car?

Why did you put on a new prop? Did you hit something and break the old prop? You might have bent the prop shaft which would cause the vibration. Wouldn't feel it at lower speeds.

you might of cracked a magnet in the motor and its hitting the windings in the motor or you mite of bent the prop shaft there is no prop adjustment

If it works leave well alone ,5.7 litre engine and you want more power ?? If you start playing with it you will defer away from the factory spec with all the possible problems of passing the annual vehicle inspection

is there a chance you have an air pocket in the flapper itself... try replacing flapper with a solid black rubber one.... they have more weight to them maybe the lever on the out side of the tank is getting caught up on the bottom of the lid

Make sure the chain to the flapper is loose when the flapper is down and that the handle when it goes through the toilet is free moving.

The props have a rubber bushing in the hub that can start to slip after time.Take the prop off and take it to a prop shop,not a dealer, and they can check it for you.The bush is there to stop damage to the g/box and driveline if something is hit.All props can be spun if the engine is off and the motor is in gear.It's another driveline saver.But the prop can only be spun backwards and thats when it clicks.Hope this helps

the inside of the prop has a rubber between the prop itself and the inside splines to act as a shock absorber,say if you were to hit a rock. that rubber had spun out and the splines are turning inside the prop. the prop is screwed. seen it 100 times before.

best thing to do is to take the intake and valve covers off, its possible you may have inadvertently sucked something up and it's in you intake. That is the best solution.

No telling what it is until you take things apart and take a look. It shouldn't be too hard, make sure you get new gaskets, gaskets are cheap anyways.