[EricH]

I seem to recall that noisemakers (such as airhorns) are not allowed. Section 3, At the Events, Game Manual, 2008. (Similar rules have been in place for years. Don't expect a change.)

[EDIT] After getting some time, a look at the manual reveals:

Originally Posted by Section 3

3.15.3 Competition Spirit
We ask that you choose to bring attention to your team in ways that are in good taste and in the
spirit of the competition. Please refrain from the following:
 Using obnoxious noisemakers.

Air horns count. Later:

Originally Posted by Section 3 again

3.17 SITE RESTRICTIONS
[...] Do not use noisy devices, such as floor stompers, whistles, or air horns