the noise that is no more ??

So i got the day off, i'm looking for a reason to take my rig for a ride, i decide to hit the coffee shop for a caffeine injection. It's raining, i put on the defrost ( so i thought ) blower to three, off i go. About two miles down the road on a snakelike curved road at about 30 mph i hear what sounds like a knocking noise.( bums me out immediately ) it does this for
about ten feet stops for about twenty feet then does it again for about another ten feet, i pull over, notice my windshield not de-fogging to notice i never put it on the defrost mode but rather i had it on the vent
mode with the vents closed. i turn it too de-frost mode continue down the road and cannot get the truck to reproduce the sound. At first i thought the sound was brake pad related or maybe i picked something up on the road that was bouncing around in my wheelwell,( passenger side front is where the sound seemed to be coming from ) now i'm wondering was my hvac system trying to tell me something like " yo
your asking me to send air through your vent system but you got the vents closed so the air has to go somewhere else " ..??????????? Any ideas anyone ?????

' 04 f-150 xlt 4x4
reg. cab flareside
drk toreador red

https://www.f150online.com/galleries....cfm?gnum=4362

 

Put it back on "vent" and see if the sound comes back.

Went away after you put it on defrost, right?

 

Ya exactly that. i'll try it in the morning. I got so miffed that i couldn't reproduce the noise that i brought it home shut it down and will deal with it in the morn. The dealer i bought it from is near where i work so if something does decide to go wrong i'll drive it right into ' em . hopefully it was just sumtin' stupid..



' 04 f-150 xlt 4x4
reg. cab flareside
drk toreador red

https://www.f150online.com/galleries...y.cfm?gnum=4362