Aha! Could this be the noise I was talking about? This guy Wheelie Pete talks about it.

http://m.youtube.com/watch?v=VeEb5KvyURI

The guy that sold me the truck said that it would need to have the valves readjusted because of the engine rebuild, which was reasonable. However, even if there was some valve noise which was fixed, maybe the similar noise that a broken driver's side timing chain guide was hiding behind the regular valve noise?

Haven't ordered the O2 sensor yet but I'm getting to it.

 

If you even suspect that your guides might be broke, I personally would stop driving it NOW until you can change it out. I bought mine and drove it 40 miles home. Checked the guides and I was about 5 miles away from wearing into the water pump passage.

It's easy to pull the valve cover and check. Peace of mind is worth 30 minutes of your time.

 

Yup, I took off the valve cover and the passenger side timing chain guide was broken. Isn't this the opposite of normal? Doesn't the driver's side guide break way more often?

Anyways, I can't stop my life just to get it fixed. I'll do it as soon as possible, but can't stop driving until then.