Does it sound like it is inside the cab out outside? Possibly your HVAC blower? Can you pin point the noise? The more detail you can provide the better we will be able to help you.

 

could be an exhaust leak. the stock exhaust runs right under there.

 

Def only
Noise from behind controls

Sounds to me that the vac hose on the back of the control came off OR the Accordion sound could be the diaphram for the diverter door has a hole

I would get under the dash, locate the diverter door diaphram, remove the hose, start the engine

see if you have any vaccumm to the diverter door diaphram hose

while disconnected the accordian sound should go away, if so then you found your problem.