My son has a 1999 Blazer with the 4.3 Vortec. All of a sudden it started making a horrible, loud clanking noise and also a squeaking noise (squeaking sounds like the belt but not sure) My husband at first thought maybe the bearings went out of the water pump, not the problem. Checked for loose torque bolts, not the problem. Any ideas for us? Truck starts fine and runs but I won't let him drive it sounding like that. BTW....it is not a rod. Oil pressure is good and it is MUCH louder than a rod knocking.

UPDATE - We are pretty sure we found the problem. Flywheel needs replaced.

 

Is there a CEL on? Is it only while turning or driving? Wonder if the exhaust is loose and apart...

 

Is the serpentine belt tensioner working properly, is the belt not too loose?

 

The sound is constant. It starts as soon as the engine starts.

 

Not sure but we will check. It started out of nowhere and my husband did mention it could be one of the pulleys.

 

Does the sound get faster (increase in frequency) with engine speed or vehicle speed?

 

The sound is constant. It starts as soon as the engine starts. I would try removing the belt; this will disconnect all items in front of the engine. Also sounds like maybe your husband did this checking water pump bearings, but did he start it? With it running and belt off, I would take a piece of short garden hose and hold one end to my ear and the other end to timing cover and different spots around the engine to see you can locate were the sounds comes from.
Also does it seem to be running on all six?

 

She updated on her first post - broken flywheel or she stated it needed replaced ....