Engine noise for normal space, tunnels and underwater. I have always used the same sound ID for all three numbers, since the other two most likely don't matter at all. You can find the sound IDs in [Data/Sound/Sound.txt]. (The ID has four numbers)
If you want to add a new engine sound to game, you have define it in [Data/Sound/Sound.txt] by adding new lines for it. An example:
9901 // 4-digit sound ID, write a new number here and use it in car's textfile.
0x00,0x01 // Type (0), Flags (0)
NewEngine.WAV// Filename of the sound file in [Data/Sound]-folder.
6000 // Priority
0 // Repeat rate
200,200 // MinVol, MaxVol
0,0 // MinPitch, MaxPitch
0,0 // MinSpeed, MaxSpeed
0x0 // Special FX index
1 // Number of low memory alternatives
5300 // Probably a sound ID to use instead of this sound, when the memory is low
Some existing values for use: (You can hear them by finding the WAV-filename in [Data/Sound]-folder)
5310 = Engine1.wav
5311 = Engine2.wav
5300 = Engine3.wav (Normal sound, Eagle 3)
5301 = Engine3E.wav
5302 = Engine3W.wav (Buzz Lightweight)
5312 = Engine4.wav (Farmer Fright)
5313 = Engine5.wav (Heinz Faust)
5314 = Diesel.wav (Trucks)
5350 = Gosiren.wav (Copcar's sound with siren)
4420 = Enginewide.wav (Wide boy's engine sound with bass)