engine noise

are they the stock sled type rockers? It does sound like one of them is pre-loaded incorrectly. I know how to set up with the cobra rockers, I did that first thing after I got my 87, so I have no idea with the stockers. But thats what it sounds like to me. Perhaps one of your lifters also isn't pumping, and that could be an issue as well.

I don't think that mixing up rockers have anything to do with the noise. If you have EFI.. i think you may want to check the injectors to make sure all of them are working good. Check the sparkplug wires too because even if one of them slides off the end of the plug it will cause the engine to make a knocking sound. Also make sure you check the exhaust header bolts and make sure the bolts are tight and check and see if the header gasket is burned out..because all of that will cause the engine to make a pinging or knocking sound. Did you torque them one cylinder at a time as you turn the motor using the engines firing order? If not it may be a preload problem causing the noise... try resetting the valves if nothing else works.

It was one or two of the lifters had some sludge buildup in them and they were not extending fully sounding like a bad valve tapping. Had it happen to me twice in the last six months since car sits over the Winter. Rather than revving it to 5K, throw some trans fluid in the oil and run it for a while then change the oil. Some of these oils today have much different contents than when these motors were made. Zinc (I believe) has been cut dramatically from oils and can cause some serious lubrication problems from what I have heard/seen. It is unfortunate, but oil manufacturers in conjunction with car manufacturers are over-engineering some things that worked in the past to reduce emissions, wear, lubricity, and the public shall suffer like in so many other areas. Best thing to do, is keep the oil clean, and buy a good detergent oil to keep things operating properly. Good Luck!