Ticking noise in granny low

[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". We've had it for decades....

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 a lot of general gearbox noise anyway when we're in a gear that low but I can hear this if all is quiet. 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 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 - 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 no
What is it? Anyone know? Theories?
thanks, rScotty