'96 Ranger Clicking Under Dash
 

1996 Ford Ranger XLT 4.0 liter engine with 185 K miles 2 wheel drive
I started the truck today a low and slow ticking sound under the dash on the left-hand side while the truck was moving.
It did not make the sound while the truck was stationary.
As the speed increased so did the clicking. There was no change in volume.
The speedometer was working during the clicking sound.
No other change in the truck's performance was noted.

Any ideas?