Whining Noise When Starting.... Pump?

Any one have any problems with a whining noise when they start there truck? I have an '05 TB and recently it has starting making a whining noise when the truck is started and it runs for about 15 - 20 seconds and then cuts out. A friend of mine listened to it and crawled under neath and he is saying it is something with an electric pump part of the exhaust? I can't remember what he called it. Does anyone have any input. Its one of these things thats really getting to me and I dont want to take it to a dealer with out knowing cause we all know they rip people off.

Welcome to AF, what part of the truck was he at when he heard this, back near the tank, up front, lt or rt? Info missing here is pertinent.

He was laying right under the drivers door area... you can hear the sound loud and clear though when you start the truck but loudest if you bend over by the drivers door.

Probably the auxiliary AIR pump.