There is probably one of several things wrong with your S10. I think @Hash_Brown is on the right track.
First, it might be an exhaust leak. It's amazing how the puffing of exhaust coming out where it's not supposed to sounds like a "tick". This can also cause a lean bank check engine code to appear. This is because it will actually draw air in as well as the puffing out of exhaust. This has to do with Bernoulli's Principle I believe.
Second, it could be a sticky lifter or valve. With as many miles on your truck as you have, this would not be out of the question. If the one or both of these are sticking, you could possible get it clear up with some SeaFoam or the like. Following the directions on the can, you can add about 1/3 of the can to the crank case and run the engine up to temperature. This will clean out the gunk from the lifters and allow the oil to flow more freely as well as allowing the lifters and valves to move around better as well. These directions may be a bit off because I have not used it in this manner in a while, so ensure you follow the directions. You do not want to leave it in your crank case for an extended period of time. Nothing past 100 miles is what I've been told in the past I'm pretty sure. It can also be used in the gas tank to help free up noisy injectors. NOTE: I will warn you, doing this also cleans all of the carbon off of the oil control rings and out from the bearings and wherever else it may be hiding. While you may think this is a good thing, as engines get older and build up the carbon, it actually helps to keep oil usage at bay. When you clean these deposits out, it will actually allow your engine to burn oil and you'll have the tell tale sign of blue smoke coming out of the tail pipe.
You could possibly try a rocker adjustment, but I would bet this is not the problem. If you have a dead lifter (one which no longer springs back), adjusting the rockers will not solve the issue. This would require you replace the lifters. At your mileage, you'd just want to do a rebuild at this point, since you'd have the top end of your engine apart anyway. Once the lifters are set, they very rarely need an adjustment. I'm assuming you have the 4.3L V6 engine (I don't think they ran the S10 with a 4-cyl and 4x4). The 4.3L is basically a Small Block Chevy (SBC Gen I) engine with two cylinders cut off. To adjust these rockers, you do not use a feeler gauge. If you've never done it before, I would get a friend who has to help you so you'll understand, but here's the basics:
- Remove your valve covers
- Move the engine so it is top dead center on the #1 piston on the compression stroke
- Both your intake and exhaust push rods should be able to spin
- Back the rocker nut off a full turn on your intake side
- While spinning the push rod between your fingers, tighten your intake rocker until the push rod just stops spinning, then give it another 1/2 of a turn
- Do the same for your exhaust rocker arm
- Then do the same for the exhaust valves of cylinders No. 5 and 6 and the intake valves of cylinders No. 2 and 3
- When done, turn the engine over (by hand with a socket wrench at the crank) 360 degrees
- Then do the same for the exhaust valves of cylinders No. 2, 3 and 4 and the intake valves of cylinders No. 4, 5 and 6
- Button the valves covers back down
You can follow this video on YouTube as it is fairly decent, but shows a V8, not your V6. (I have timed the video to 1'6" which is the meat and potatoes portion of what you need to be watching, but go back and watch the whole thing if you like.)
Third and ugliest is the possibility of a rod knock. I'm doubting this from your description. The noise would be a bit deeper in pitch than a tick, but it can show up sounding like this.
