It's pretty hard to diagnose a noise over the Internet without hearing it. I would at least check you oil pressure and oil condition after a drive like that.
There is a TSB about knocking on that engine but I don't know if it would apply to you.
Here it is
File In Section: 06 - Engine/Propulsion System
Bulletin No: 99-06-01-003
Date: April, 1999
Engine Bearing Knock Noise,
(Re-support Crankshaft, Select-fit Undersize Connecting Rod Bearings)
1999 Cadillac Escalade
1996-99 Chevrolet and GMC C/K, G, P Models
with 5.0L or 5.7L Engine (VINs M, R - RPOs L30, L31)
Important :GM Canada Dealers require prior DSM authorization to apply this bulletin.
Some customers may comment about an engine "knocking" noise.
A condition may exist in some engines where, the crankshaft is NOT being evenly supported by all five crankshaft bearing inserts. In these engines, the number 1, 2, 3, and 4 crankshaft bearing inserts are supporting the crankshaft, and the number 5 crankshaft journal (rear) has excessive clearance relative to the number 5 crankshaft (main) bearing cap insert. In this condition, the crankshaft flexes under load, and pounds on the lower number 5 crankshaft (main) bearing insert creating the knocking sound. The engines were originally built with 0.0006 in. Undersize crankshaft (main) bearing inserts in the number 2, 3, and 4 crankshaft (main) bearing locations and 0.001 in. Undersize insert in the number 5 crankshaft (main) bearing location. The service procedure listed below addresses the above condition by lowering the crankshaft at the number 2, 3, and 4 crankshaft (main) bearing positions (increased crankshaft (main) bearing size of the lower crankshaft (main) bearing inserts) and raising the number 5 lower crankshaft (main) bearing insert (undersized insert) in order to properly contact the number 5 crankshaft journal surface. These engines may also exhibit a connecting rod knocking sound. In these engines, the connecting rod knocking sound is caused by excessive connecting rod bearing clearance.
A customer concern vehicle may exhibit one or both of the above stated conditions.
Follow strategy-based diagnostics for engine noise listed in the front of Section 6 of the Service Manual.
Some additional key points:
Rod Bearing Knock - Occurs on initial engine start-up, and can also be heard as high as 1,500-1,800 RPM. Rod bearing knock typically diminishes or completely goes away when the engine reaches normal operating temperature.
Crankshaft Bearing Knock - Crankshaft (main) bearing knock sounds deeper in the engine and also sounds more muffled. There are two different types of crankshaft bearing knock.
Short duration cold knock typically occurs for 1-5 seconds on engine cold start-up only, and almost always occurs on vehicles equipped with an engine oil cooler.
Hot knock occurs less frequently, typically occurring in very hot ambient temperatures, and can be heard up to 2,000 RPM.
Technicians can increase the likelihood of reproducing bearing induced knock sounds by slightly depressing the accelerator pedal while starting the engine.
Monday, December 20th, 2010 AT 9:18 PM