This one has me stumped as I thought it was my worn out brakes doing this but I just completely redid my brakes with new pads/rotors and its still there. For the last few years I have been hearing a sound that I can only describe as a "fluttering series of pops" like something sticking maybe and working its way out or in. It is engine related because I have only heard it while the truck is running and I have heard it while it is parked or stopped. Thought maybe clutch related but I see no rhythm to it and what the clutch is doing at the time. However if the heater or defroster is on, it does appear to make it more frequently and as the air temp cools like it is now, it tends to pick up. So maybe vacuum related....

From inside, it sounds to be coming from the passenger side under the hood but the sound is REALLY hard to pinpoint due to it frequency and volume - loud and low. You hear it when the truck approaches and the heater is on easily.

We're talking 02 F150 4.6 5speed 4x4 Sport by the way.

I hate these kind of vague a$$ questions but I have some hope somebody says "Ooh yeah, happened to me....." because I don't know where to go from here.

Last night I tried to hunt it down but it got dark fast. It appeared to be coming from the K&N filter. I pulled up on it and it still made the noise so I don't think something is hitting it but I am suspicious now. Tried this morning but it didn't want to do it.

Also I can confirm:
Cold makes it worse
The blower fan makes it worse
I can't trace it to a pattern with the brakes or clutch operation
As the truck warms up it becomes less intense
It's tied to the engine as it affects idle

 

After getting some road miles on it to verify I fixed it, it was the Idle Air Control Valve. Took it off to clean it on some advice and it lessened to only return. Bought a new one and problem solved.