I just wanted to step in and correct a factual error and provide some accurate rules information. As far as I can tell, noisemakers of any type, including the air-horns on trailers, are NOT a violation of UIL rules. The rules guidelines quoted earlier from the UIL website were taken from the BASKETBALL manual. The Constitution and Contest Rules section on Athletic Regulations (http://www.uiltexas.org/files/constitution/uil-ccr-section-1207-1210.pdf), which applies to all sports, simply says:
MASCOTS, FIREARMS, PYROTECHNIC DEVICES, BALLOONS. Live animal mascots, cannons, firearms, fireworks, etc. are prohibited at League athletic contests. Except in their normal course of business, emergency vehicles shall not flash lights or sound sirens during the course of a game. Noisemakers are prohibited at League athletic events held in field houses and/or gymnasiums. Releasing balloons is prohibited at League athletic events held outdoors. [Section 1208(n)]
Based on that, since football is not held in a field house or gymnasium, hand-held air horn, propane tanks filled with ball bearings, and even air-compressor driven air horns on trailers are all allowed. Only emergency vehicle sirens are prohibited, and then only during the course of the game (I've seen ambulances at Vidor flash lights and turn on their sirens as their team enters before each half).
There are two football playing rules (http://www.ncaapublications.com/productdownloads/FR09.pdf) that could have an effect, both of which have already been mentioned. One has to do with safety:
RULE 1-2-8 - Markers or Obstructions
a. All markers and obstructions within the playing enclosure shall be placed or constructed in such a manner as to avoid any possible hazard to players. This includes anything dangerous to anyone at the limit lines. The referee shall order removed any hazardous obstructions or markers located inside the limit lines.
b. Any markers or obstructions within the playing enclosure but outside the limit lines constituting a hazard shall be reported to game management by the referee. Final determination of corrective action shall be the responsibility of game management.
Note that the "limit lines" are are the dashed lines around the sidelines that create a 12-foot border around the field, or at least 6 feet if space won't permit the full 12 feet (example, due to a track). I'm not sure the dimensions of our field and how the limit lines are marked.
The other rule has to do with noise:
Rule 9-2-1-b-6
Persons subject to the rules, including bands, shall not create any noise that prohibits a team from hearing its signals (Rule 1-1-6).
Rule 1-1-6-b
All players, substitutes, replaced players, coaches, trainers, cheerleaders in uniform, band members in uniform, mascots in uniform, commercial mascots, public-address announcers, audio and video system operators, and other persons affiliated with the teams or institutions are subject to the rules and are governed by the decisions of the officials.
Fans are considered "affiliated with the team or institution" and are subject to the rules. For instance, the home team can be penalized for fans entering the field prior to the game having been declared ended (surely some of you have seen that in college football). The penalty for violating Rule 9-2-1-b-6 on noise is a 15-yard unsportsmanlike conduct penalty. However, I've NEVER seen that enforced.