I'd check three things: first, check your house's water pressure, then, remove and check the cap seal, then make sure that the refill hose is properly attached to the clip above the top of the overflow tube.
You might also verify that the float moves freely. The reason to check the house's water pressure is a toilet valve is probably one of the weakest things, and if the pressure is excessive, it can open on its own, momentarily release the excess pressure, then close, then open again. The pressure should be below 80psi.
This replaces the rubber seal.