Any noise will work, as long as you stand close to where the food usually gets dropped.

They could probably even stand a banjo(?), but I'm not sure a p'whistle would be within their auditory response range, unless you used one of the long ones. I've known farmers who could "whistle them in" but most small herds (that I've seen called to supper) respond quickest to the voice-range "cattle calls" they're used to.

When they're not busy with something important, cows can be quite curious about anything "unusual" - most pasture environs seem somewhat lacking in intellectual stimulants - and a whole herd will tend to follow along if one of the "lead cows" starts walking away from the crowd, so you really only need one "music lover" (or critic?) in the herd to get the effect shown.

John