2006 F350 V10 and what was a random noise after sitting for long periods and starting has become a much more common noise, start the truck and just when you're ready to put it in gear it makes a snarling noise for 1/2 a second that first few times I thought was a cat in the radiator fan. It is completely random when it will do it now hot or cold and sounds like it's coming from the front passenger side of the truck or possibly in the center and haven't been able to have it produce the sound with me outside the truck.
After listening to the sound hundreds of times and comparing it to other things it sounds to me a lot like air in a high-pressure oil system, almost the same sound as my farm tractors with air in the hydraulic systems....
No leaks and all fluids are full and clean, The truck runs like new other than the random start up noise and I don't like not knowing what going on exactly on anything mechanical!

 

idler wheel for the serpentine belt. starting to go bad.
or tensioner wheel for same.

 

Not a bad suggestion but they are all fine, it's not a squeak or small growl but one you can feel through the seat for literally half a second and it's done.

 

My alternator did this.......right before it seized and tore the fan and every pulley off the front of the engine.

 

Pull the belt and spin everything by hand. Also start it up for a second without the belt to be sure it goes away. If it does, its something running on that belt for sure.

 

Does it make the sound with your HVAC turned completely off?

My AC compressor clutch makes quite the racket on the first start of the day.
After that it never makes a peep again until the next day.

 

I hadn't considered the AC compressor, that would also be somewhat in line with where I think the sound is coming from.... Hard to tell from in the cab as sounds don't always travel how you would think. It really sounds from the seat like it's coming from the passenger side under the firewall and it's definitely not coolant bubbling in the heater core.

I haven't driven the truck in several days and it rained this afternoon so hopefully tomorrow I'll have time to do a little testing, first I'll pull the belt off and check everything and go with testing from there.

 

check to see if you have normal cabin heat

if you dont......after the engine has been sitting over night..go down and slowly unscrew the engine oil drain plug with the goal of getting to the last 2 or 3 threads with out pulling the plug....check to see if water starts to drip before the oil drain plug is removed.

if you do....quite possible of head gasket leak ....cumbustion chamber into cooling system.