It's not in the Rules Book because there is no rule about noise in the NFHS rules. However the state does have a regulation which I found on Page 113 of the VHSL handbook. (You can get the handbook as a pdf from vhsl.org)
The part the covers air horns is quoted here:
The use of artificially and/or amplified noisemakers (examples of this include, but are not limited to, train or diesel horns, amplified guitar note or riff ) are not allowed during live ball situations at any VHSL event. Any deviation from this policy will constitute a VHSL Sportsmanship Violation (Excessive Unsportsmanlike Conduct - $300 Fine). Note: For the purpose of this rule in football, live ball situations begin with the ball being handled by the snapper.