Ticking sound

[Aqualight]

I have noticed a ticking sound coming from the passenger side towards the front of the truck. It seems to be speed sensitive. The faster I go the faster the "tick". It doesn't appear to be engine related because I can let off the gas and the ticks remain. Almost sounds like a rock or something lodged in the tire. I have looked at the tires but can't see anything that might be stuck in the treads. I have tried cycling the 4 wheel drive settings but no change.

Any ideas?

Thanks in advance