My 2004 chevy trailblazer is knocking

Could be a big end or main bearing but there are a few simple checks that can be done first
1--have the oil pressure checked using a mechanical gauge
if there is a bad bearing there will be low pressure at idle
2 --check the exhaust for smoke under load
if there is a bad bearing , the escaping oil will overload the oil ring and produce smoke
( if smoke comes after idling at lights and on taking off that is valve stem seals)
3--- run the fault codes and check for knock sensor problems
the noise could be detonation under load from lean fuel mixture or incorrect timing
4---check the ignition timing
5-- run the engine when it is knocking and remove a plug wire and see if the knocking stops
bearings will stop knocking if there is no combustion to place load on the crankshaft
That will also indicate which big end you have to check if you have to pull the sump
Too far advanced will cause the pistons to slap the cylinder walls hard for too far advanced ignition
you statement that the noise goes after 40 indicates that it is not a bearing as the high the rpm the louder the noise
taking the foot off the pedal indicates detonation
lastly if all the checks prove good , you could always have the sump removed and check each big end and main individually for damage
sometimes a failing cam chain if fitted ( no motor identified) will knock as the chain hits the timing cover