Warning horn?

[RoyalsCoach]

I have been out of coaching this year so I have not been on this forum often nor did I buy a rule book. However, I have a question. I substituted for our regular timer the other night. A player received his 5th foul. I was under the impression that I still blew the warning horn 15 seconds prior to the end of the time to replace the player. I had the official tell me I should not have done that plus another coach. I looked in my 2007-2008 rule and it said I should blow a warning horn. NFHS site did not show the rule had been changed. Who is correct? Is there a warning horn for the 20 second replacement of a disqualified player?