Ticking Sound in granny low...what is it?

[rScotty]

Our truck is a '70 chevy 4x4 with a 4 speed manual tranny and like so many Chevy pickups it has the typical manual 4 speed non-syncro, very low, and seldom used 1st gear. It's what's called a "granny gear".

I was showing off the truck to a friend and noticed that in that gear I can hear a clicking or ticking noise about twice a second as we go forward slowly on pavement in 2wd mode. It's only in that gear, but it's very definite and very regular. Maybe as some gear revolves? There's the standard amount of general gearbox noise anyway when we're in a gear that low but I can hear this clicking easily. It's not a loud heavy sound, but you can't miss it. Definite. About as loud as a rock in a tire tread, but this only happens going forward - noise isn't there in reverse. There's either none or very little accompanying vibration when it clicks - not enough to feel through the wheel, seat, or body - although maybe I can just maybe feel it with a finger on the shifter. Basically it's just a soft click-click sound about twice a second as we go forward in low.

Unfortunately we don't know how long it has been making this sound in granny low. That gear is rarely used anyway - for us it might be years between uses for a gear that low. And I do tend to loan the truck out. Everything else seems fine/unchanged. Might have been that way for years...or maybe not.
What is it? Anyone know? Theories?
thanks, rScotty