One of the "features" of the P-51 was a tail warning radar that was supposed to let pilots know if an enemy fighter was on your tail.
However, a P-51 pilot told me that they generally disabled them.
According to him, the problem was that, if you were flying in formation and performing a turn, there was generally a point when a squadron-mate would be behind you. When that happened, a loud horn would sound and a bright light would flash in your eyes, scaring the bejesus out of you.
Has anyone else heard of that problem?
You are right, it was probably a bell - I heard the story 35 years ago. And the fact it had volume controls indicate that he would have turned the TWR down/off rather than disconnecting it.
I have read mixed reviews regarding the gunsight. Some of the favorable reviews were from old-timers who I thought would hate it.
For those who doubt that the P-51D had TWR, here is a picture from the POH showing the controls for the TWR (aka Rear Warning Radar), which are referenced in the comments below - item 8.