Engine noise for normal space, tunnels and underwater. I have always used the same sound ID for all three numbers. You can find the sound IDs in data/sound/sound.txt. (The ID has usually four numbers)

If you want to add a new engine sound to game, you have define it in data/sound/sound.txt by adding a new line for it. Let's take an example:

5311		// Sound ID, write a new number here and use it in car's textfile.
0x00,0x01	// Type (0), Flags (0)
ENGINE2.WAV	// File name in data/sound - folder for the sound, for example NEWSOUND.WAV.
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		// ?

And here some existing values for use: (You can hear them when you find the name 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)