advice on rod knocking please

I have a 1999 Suburban 4x4 with a 5.7 engine and 150,000 miles that I have owned sense new. I took it to Pro-Gear in San Diego to have the rear end fixed. They put it in the air and ran it to find the bearing noise problem when they and I heard a noise coming from the lower part of the engine. It is a tapping or knocking sound. The faster the engine went the faster the tapping was. I took it right home and change the oil and filter (mobile 10-30) and added two quarts Lucas additive to the oil. The magnet oil plug had very small black metal on it. I normal change the oil every 4,000 miles or so. I used the oil additives to prolong the engine life over the weekend. Could this knock or tapping be piston slap? Is it reasonable to think about dropping the pan and replacing the rod bearing if that is what it is or with the knocking is the crank gone too? I cannot hear the knocking with the engine running and hood open from the topside. Can the pan be dropped with out lifting the engine or dropping the 4x4 axle out? Any idea how long it will run this way? I am think about a crate engine replacement when this one blows. Thanks for any ideas on the engine. Dave

In the new engine change the oil every 3,000 miles after break in, not "4,000 or so". Change the oil every 2,500-3,000 miles from now on on this engine, it should still have some life left in it.

If it's like a tapping or clicking noise, then probably a lifter. If it's heavier knocking which goes away after engine warms up, then it's probably piston slap, which is common for Chevy engines. If it knocks (heavy) all the time, warm or cold, then it could very well be a rod bearing or possibly just bad piston slap. The piston slap will definitely decrease as the engine warms though, so if it's piston slap, it would be very loud at cold start, then mildly loud (or quiet) after warm.

150,000 miles is getting up there i would start saving for a crate engine and just drive it till the knock gets worse then swap the engine

One way to see if problem is isolated to one cylinder and not a lifter noise problem is to either disconnect one spark plug or if its got injectors for each cylinder, unplug one injector at a time. if the noise drops down or goes away then you have found your bad cylinder. If the noise never changes then its probably just a lifter.