Amtrak P40 Noise - what is it?

[spinfire]

When I listed to the Downeaster pull away from a station stop I hear a loud "whirr" noise as the locomotive reaches around 5-10mph. It sounds like a motor rapidly spinning up, but the locomotive is already moving. What is it?

I'm pretty sure the downeaster is being pulled by P40/AMD-103 locos.

Thanks.

[Komachi]

HEP generator, maybe?

[ATK]

Probably the equipment blowers kicking in to provide cooling to the traction motors. No need to provide cooling to the TM's when your standing still.

[AmtrakFan]

It comes from the Prime Mover it is a clicking sound.

[DutchRailnut]

They don't use P40's however but P42's, only P40's in service are on MNCR.
The whoop sound like submarine alarm is aircompressor starting, only thing I can think of is the Equipment blowers.