Strange clicking sound - only over 40 mph

This sound is relatively new - maybe a month or two. There is a clicking sound when I drive the truck. At first I thought it was my keys swinging and hitting something. But it's not. It's apparently coming from inside the dash to the outside of the steering wheel. Like a relay clicking but not always regular. And it only happens when the road speed is over 40 mph. Below 40 mph there is not a single peep out of it. And once over 40 mph the road speed does not alter either the sound or the pace of it. It's not fast and is sort of erratic. Several clicks and then a pause but in no real pattern. The more I listen to it the more it sounds like a relay clicking either in or out. But . . . I really have no idea what it might be. <g>

Any thoughts? Ideas on what it might possibly be?

Thanks.

PHM
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Just to followup -

This noise was due to the LF plastic inner fender liner panel having a screw missing. Apparently above 40 mph it would flap and intermittently make the "relay chattering" sound. <g> A few small screws resolved it.

 

thanks for the update