92 Bravada 4.3 vortec Valve tapping

Hi all. I have a 1992 Bravada with a 4.3L Vortec. Just got it and now thinking I got ripped off, maybe someone here can help. Seems to run good with exception of tapping coming from a valve it sounds like. coming from top of engine. goes away sometimes, and can't hear it at crusing speed, Mainly at idle and during acceleration. Was told when I bought it that the motor had been rebuilt due to being overheated badly. Rebuild was approx 3800 mile ago. The guy said that after being rebuilt it had break in oil in it and no problems. Changed break in oil at 800 mi after rebuild but he said that the valves would need to be adjusted after the break in period and he never had it done so thats why it was tapping. He said tapping started approx 1000 miles after break in oil was changed. Check engine light is also on and when i pulled the code it was code 43 which I believe is the knock sensor from what i've read. I'm not a mechanic so i don't know if valves go out of adjustment after rebuild or not maybe some one here can help.

If the valves were adjusted good to start with they should not go out of adjustment.
How ever you need to check and see if it is just a lose or bad rocker arm or bad push rod.
Pull the valve cover and have a look in there.
Can the noise be heard sitting still motor at idle or speeded up just a little?

It can be heard at idle almost all the time but sometimes seems to mysteriously almost disappear. Very noticible when engine is cold but quiets alot as oil warms up. Can be heard during acceleration but not at highway speed. I'm worried this may be a $500 or more fix. Thought maybe i may get lucky with a $250 valve adjust. What damage can this do if not corrected.

Before you can figure what it will cost you need to find out what it is.
Is the person that rebuilt engine handy to have a look at it?
MT

No. Guy I bought it from was over an hour drive away. Not sure if this is something I could possibly tackle myself. I am mechanically inclined and know how to do some repairs but don't want to get in over my head. Not alot of room to work on these engines. I have had a vehicle in the past that had a Chevy 2.3 v6 i believe, that had a spun bearing and a rod knocking and the noise was MUCH louder and a different type of noise than this, so i don't think that is it but
I could be wrong.

There may be a faulty valve lifter, push rod, rocker arm, or improper torque on the rocker arm nut. The valve lifters are hydraulic and there is no adjustment. The crankshaft must be positioned 60 degrees either direction of the index tab on the front engine cover before torqueing the rocker arm nuts. If they didn't do this correctly, the lifters will make noise. Torque spec is 18 foot-pounds. Hope this helps.

Ok this is probably a stupid question but now im nervous that it may be a rod. How can you tell the difference between valve tapping and a rod knocking. I have to drive this thing and am terrified that ill throw a rod out the block or destroy the engine.

Valve train noise is a higher pitched tapping noise that comes from the top of the engine. Connecting rod and crankshaft main bearing noise comes from the oil pan area and is more of a deeper "thumping" type noise. That's the best I can describe it.

You usually can short out the cylinder plug wires and a rod knock will change.
Lifter will keep on tapping.
If you are unsure or in dought have a trained ear listen to it.
Good luck
MT