How do I make a body position not shift the part down each time?

i am probably being a retard with body position but idk how to use lmao, but i want to make a part hover up and down like its floating so i thought that a bp will work good, and it seems like it does the only issue is that it shifts down instead of going to its default position, i speeded up the delay so i can show you what i mean in the below gyazo

local bp = script.Parent.BodyPosition

while true do
    bp.Position = Vector3.new(script.Parent.Position.X, script.Parent.Position.Y  + 0.400, script.Parent.Position.Z)
    wait(3) 
    bp.Position = Vector3.new(script.Parent.Position.X, script.Parent.Position.Y  - 0.400, script.Parent.Position.Z)
    wait(3)
end

https://gyazo.com/011c018e515146e63b588d562a0818dd

ty if you can help me!