Hi guys, Im new to this forum. I just bought a 1996 Chevy Blazer LS aobut a month ago.
To get right into it, I believe Im having problems with it already. It has a replaced engine with appx. 104,000 miles on it. I hear this noise at idle but not when getting up to speed, it sounds like a rod knock. It doesnt sound "metallicy" its more of a deep sounding thud but its constant and doesnt change pitch which gets me to believe its the flywheel. So I did the test for rod knock by pulling the spark plug wires one at a time to see if it quieted down; no cigar, it just kept "thudding". I know its coming from the bottom end of the engine, when I went under there I felt underneath the oil pan and for every knock i would feel vibration on the pan. Anyone know what this noise is? Could it be the oil pump?? Thanks guys!

 

Does it increase in speed when you increase the engine RPM?

 

yes, but only until about 1200-1300RPM that I cant hear it. any ideas?

 

If it increases with RPM and can be felt through the pan, it likely is in your bottom end. How is low RPM oil pressure? Have you put it on an actual mechanical gauge?

 

I have a similar problem Check this thread and see if its what your going thru?

https://blazerforum.com/forum/engine-transmission-35/2000-blazer-4-3-knock-idle-warm-48084/

 

the oil pressure is awesome, between 40-60 cold idle and at warm idle its at about 30. the weird thing is is ive been driving it for over 1,000 miles now, shouldnt it have thrown a connecting rod already?

 

If it starts to get louder and worse then i would say its a bearing in the bottom end. Just be cautious drive it and see what happens.

 

possibly a thrust bearing that has worn out? The thudding could be your whole crankshaft sliding back and forth.