During the 30-second TO for replacing a disqualified/injured player, is there a warning horn, and if so, when?

No warning horn. After letting the coach know he has to
replace a disqualified player instruct the timer to let you know when 30 sec is over.

To avoid any problems with the coach not getting a sub in within the 30 second time limit...I have the timer tell me when we reach 20 seconds, so that I can tell the coach that there is only 10 seconds left.

I agree that a coach has a 30 seconds with no warning horn. But, Dennis is not wrong in my book. Instructing a timer to give a warning at 20 seconds is good preventive officiating!

AK ref SE

Why not use a warning horn?
 

Have the timer sound a warning horn, then there is no doubt to anyone in the place that his time is running out.

My Two Cents Worth
 

I really don't have any preference but here is why I believe there should be no warning horn.

#1. The warning horn in all other situations indicates to the referees and to the table that no player may be allowed to enter the game (during a time-out or end of quarter). I don't think using the warning horn as a "you're running out of time" warning would pose a problem but you never know.

#2. The coach should be looking to find a replacement immediately. Many of the coaches I know would not replace the player until the horn went off. Leave the coaches guessing as to how much time is left! - Again, I don't see much of a problem with this.

Just my two cents worth about possible reasons that there is no warning horn.

Confusion on situation
 

What gets me is that the rulebook says nothing about a warning horn, yet the "instructions to timers" sheet (by the NFHS) says sound a horn at 20 sec.

Are you sure that is not for the new 30 sec time out.

Even more confusion
 

The NFHS instructions say just sound horn at the end.

The IAABO instructions say "warning signal" with ten seconds left.

The other night, I had team A's coach try to tell my partner and I that team B's coach was not allowed to have his players huddle near their bench while the 30 second clock was running after a player had picked up her 5th foul.

I told him that there was no such rule and that I was 99.99% sure that that was not the case. As long as the remaing four (4) players stayed on the court, he could talk to them.