My 200k+ blazer recently started knocking.
The sount intensifies most in reverse and least in neutral and park. But when it's warmed it it knocks in any or no gear at all.

I have driven it for 700 miles now since it started knocking. I was told it would have thrown a rod if that was the cause of the knocks. It still runs fine although I don't rev it past 1500 much and never above 2000.

How do I confirm if it's Indeed a rod knock? I don't hear the noise when driving fast, but it does increase in frequency with higher RPMs.

Also, if it is the flex plate doing it, what can I expect to happen if I keep driving without fixing it?

Thanks

 

A broken flex plate will normally make a “clicking “ sound. If it truly is a knock I would suspect a rod. Just because it hasn’t made a window to get out isn’t a good way to diagnose. Oil pressure will be low with a bad main or rod. And a broken flex plate will leave you sitting along the road eventually.
Brad

 

Mine had a broken flexplate and it sounded very much like an engine knock. I ended up replacing the motor and didn't need to. By the way it is easier to pull the tranny than the motor. I even looked in at the flex plate before all that happened and didn't see anything. It ended up being broke real close to the bolts so I couldn't even see. I would make sure that it isn't the flex plate.

 

G'day, An acquaintance of ours had a really nice rust free 1976 K10 with a lift, aluminum rims with 33's, and a fairly fresh 400 with a knock. The shop he took it too was the same one who had done the motor and they would not warrantee it. So it was for sale cheap. I bought it sight unseen and drove up north the next weekend to check it out. Well, the motor was fine but the flex plate was not. He had been driving it so long that the vibration had caused the six bolts holding the bell housing to the block to come loose and the transmission case was shot. We pulled the motor and put a used external balance flex plate on it and threw it on the test fire stand. It was a beast and ran quite well. So I sold the engine to someone starting out with an entry level stock car and parted the truck out.

So yes, a flex plate can sound just like a rod knocking. It can even fool a mechanic. Of course if they had looked at the transmission ready to drop they would have diagnosed it differently.

 

For my experience with rod knock - happened once.

It can intensify with the load and AFAIK never goes away completely.
Might be worth having a look at the flexplate.

Another thing that comes to mind which makes a knock-like sound is the exhaust gaskets or the headers when cracked around an exhaust port in the heads.
a loose spark plug may do the same but then it's more like a chugging sound.