They're simple to make. First you need a spinner with two holes near the center. Just about anything will do -- a button, a piece of cardboard or plastic, a sliver of old bone. The Gunbatau of Alung-Bewai used old credit cards, which they gathered from trash containers along the outskirts of Chulum-Sanda. Next, run a loop of thread or string, about 2-3' long, through the holes so the spinner is somewhere in the middle. Drape one end of the loop over one finger on each hand, pull it tight, wind it up a few times, then give it a tug. With practice, you can get the spinner spinning back and forth to make a noise that sounds almost, but not quite, like something that has nothing whatsoever to do with bulls.
You can make these things just about any size, ranging from tiny 'gerbil-roarers', fashioned from shirt buttons and bits of thread, to massive 'extinct-mammoth-roarers', fashioned from manhole covers and lengths of rope, operated by teams of stalwart youths hauling on each end. Alternatively, if stalwart youths are not available, you can use somewhat larger teams of lingerie models, though this can lead to problems under some circumstances.
Hoping this has been of some use...
DesertPilot
P.S. There are no such things as the Gunbatau, Alung-Bewai, or Chulum-Sanda. I made that part up. But I had you reaching for Google for a second there, didn't I?