Question about loco noise

[efftenxrfe]

Air pump/compressors start when a main reservoir's air drops to a preset value. Close by the main res a switch controlled by air pressure starts or permits the pump to do its thing.by sending  electrically a signal simultaneously thru the MU cable to any  other unit's pumps which together using the (MU) main reservoir equalizing pipes get all the res.'s pressure to another preset pressure, usually about 10 psi difference low to high.

Not having viewed the clip my thought is that the pumps started because ancillary use lowered the MR pressure....like sanding, bell-ringing, whisteling, recharging and compensating for brake pipe leakage, windshield wipers too.

During train brake release pumping is usually continuous until after release and recharge of the train brakes. is completed or, for recharge, nearly so.

another nit plucked. 

[bubbajustin]

This is often hered just as a train begins to move, because the compressors are begining to charge the air tanks again after relesing the breaks.

[YoHo1975]

We really need to have a Locomotive FAQ at the top of this forum answering questions like this.

No offense intended to you Jeremy, but this is a very common question. 

[JeremyB]

Thanks for explaining that Ed

[edbenton]

What it is simply is the Air Compressor Kicking in.  Ge uses a Motor Servo Booster to kick in their Electric Motor Driven Air Compressors and the Whomp noise is the Motor Servo telling the Compressor to GET IT GOING.  Think of it as the Drill Instructor going up to a Recruit and putting their GI Issue BOOT into the Recruits REAR END.  Be amazed how much motivation it gives. 

[JeremyB]

Hi Guys

In this video at the 1:45 mark can anybody tell me what that sound is?

http://www.youtube.com/watch?v=PoQso-zjZsE&feature=related

this is not my video but Its the one I found where I can hear that sound.

Thanks

Jeremy