Could be a couple of things.
1. The codes used to require a short riser (nipple) of pipe at the stop valve at each plumbing fixture called a hammer arrestor, which would cushion the hydraulic shock of the water finding the end of the water supply pipe, causing what is referred to as air hammer. They are rarely used anymore. Sometimes, even when used if the pipe isn't properly strapped, they will vibrate or slap against a stud or wallboard when a plumbing fixture is activated.
2. If the float arm has a float ball at the end, you probably have the older style ballcock or water control valve in your toilet, This valve functions much like a faucet in that at the top of the valve where the float rod connects to the valve inside, it compresses a washer against a brass or plastic seat stopping the water flow into the tank when in its raised or full position. The washer may be loose and fluttering, causing a sound similar to NV when he farts.
In days of olde when knights were bold and rubbers weren't invented you'd tie a sock around your cock, and babies were prevented