Truck Groans? when starting..

[Brian82]

Ok, not really sure how to describe this. have heard this a few times while starting the truck in the morning, maybe 70 degrees out, not cold by any means.
It ..... groans, or almost sounds like there is an noisy electric motor rotating when i first start the truck, goes away with-in 3-4 seconds. Not related to the starter motor rotating.

Any thoughts?