Real diesel engine often emit hissing and popping sounds (there is also a crackling sound like static electricity being zapped) while stationary on the platform, which someone said are the main air res. psi safty valve venting air..and the spitter valve "spitting" moisture out of the brake system. If this is true source or not, I do not know, but do any model RR diesel engines emulate these sounds (in addition to horn, PFA, whistle, etc.)
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I know that the MTH Premier SD45T-2s and SD70ACe that I have make those very noises you refer to. Very realistic.
quote:which someone said are the main air res. psi safety valve venting air..and the spitter valve "spitting" moisture out of the brake system.
That is correct. The sounds really got on my nerves while sitting still.
My Lionel Legacy diesels all produce a wonderful variety of miscellaneous sounds. Both when idling, and at random times when running. 
I have a couple Proto 1 , a diesel and steamer that make clicking and static noises during start up but stop once there idleing.
The spitting/popping noises on real locomotives are main reservoir automatic blow-down valves, which prevent moisture from accumulating in the air tanks. Modern versions are activated by an electric solenoid triggered by a timer circuit. Sometimes they malfunction and spit too often or repeatedly. For model railroad sound effects, I find them to be annoying if they are too frequent. Twice a minute at idle would be enough IMO.
I remember that some of the older diesel locomotives had a manual blowdown control button on the control stand in the cab.
hissing and popping sounds (there is also a crackling sound like static electricity being zapped)
Come to think of it, I don't want any of these sounds on my O guage stuff, at least...... not unless its still under warranty.
The recent Lionel EMD Demo SD70ACe has the popping sounds at idle. I dont believe their earlier SD70ACe loco had this feature, I actually find it rather annoying and would prefer not having the sound.
The spitting/popping noises on real locomotives are main reservoir automatic blow-down valves, which prevent moisture from accumulating in the air tanks. They are activated by an electric solenoid triggered by a timer circuit.
Some 20 years ago, a supplier came out with a very small "steam trap" design for condensate draining. Thus, the continuous "popping" or "spitting" sounds you hear are from these little steam traps mounted on each main reservoir, in order to drain condensation. There is NO electrical connection nor electrical activation for these steam traps to operate, in fact the unit can be COMPLETELY dead and pushed into the diesel shop, and so long as there is ANY air pressure in the reservoirs, these darned things will "spit & pop"!
The modern electric blow-down valves have electric heaters so they won't freeze up in winter.
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