About a week ago I noticed a rather loud and consistent knock while at idle. Its only at the 500-600 rpm range and disappears entirely if runs any faster. To me the knock doesn't sound very internal and when the knocking is going on I noticed the belt tensioner was moving also. I swapped it out hoping to solve the problem and the new one is moving right on along with knock so it must be elsewhere. Curious if anyone has dealt with this before and where I should start troubleshooting next.
Thank you.

I have a 1999 4 door S10 blazer, 4.3 with 250k miles.

 

pull the belt off and fire it up and see if noise is gone, If so check the belt for wear or missing pieces. Also check all things that the belt runs, such as power steering, alt, water pump ECT for dragging bearings as you spin the pulleys on them or back and forth slop when the pulleys are wiggled side to side

 

Ok, with the belt off there is still a knocking at 500-600 RPM, so that ruled out a ton, thank you. Harmonic balancer maybe? I know I'm stuck on the belt system but something really had the belt system acting choppy when the knocking is happening. Forgive my terminology also, I'm an amateur tech haha.

 

It's poss that the harmonic is causing it, Years ago we had a 231 Buick v6 that had the center bolt work loose and made a knock that sounded like a rod knock at idle.

Check the center bolt and the pulley bolts, look for shiny spots around bolts, and make sure they are tight. Also carefully with it running look for wobble in it.

As you may know the harmonic is made out of two pieces of steel with rubber inbetween and the outer can slip over the years throwing the engine out of balance.

If you deside to swap out the harmonic use the right pullers to remove it and when reinstalling it [DO NOT and I repeat DO NOT ] beat on it or use the center bolt to pull it into place. Most auto parts stores that have a rental program will have the right press to install it in their rental / loan out program

 

Post a video of the noise. Is it a clacking sound or is it a knock. Also what is your oil pressure when fully hot at idle? What weight of oil? Does knock happen at idle after engine is first warmed up (to >170°) - or do you have to drive it a while before knock happens?

 

My oil pressure warmed up at idle is about 30. I use 5w30 conventional. I have noticed that the knock/clack is louder after being warmed up. Also it gets louder at idle when the A/C compressor engages. One more observation is that it is much louder in drive/reverse while stopped. Anything over idle there is no noise. Dont know whether to call it a knock or a clack, though it does sound external and the louder it gets the sharper it gets, so guess you could call it a clack.

 

Does it sound like this?

 

Not really, its more of a clanky sound and it doesnt pick up with acceleration it just stops. Almost as if something is wobbling but smooths out outside of those RPMs. I'm gonna go out today and try to get a decent video for you guys.

 

Cracked flex plate

 

Sure enough, you're right on that one newguy, I got a real good chance to look into things today and that's exactly the root of it. Thank you.