help me diagnose this clunking noise

[MannyDantyla]

I can't figure out where this noise is coming. First I'll describe it.

• very low in frequency. not a squeel, more like a clunking noise.
• intermittent. Can be felt/heard in the cab maybe half the time or less.
• only at idle (or can't hear it above idle because of the exhaust)
• sounds like it is possibly coming from the block, but it's not very obvious, but I think it's more pronounced on the driver's side
• it can be felt in the cab, but it's definitely audible in the engine bay
• Engine is a 350 chevy. Not original to the truck I believe, still investigating that bit.

Just from watching youtube videos, it almost sounds like rod knock but it doesn't have that slapping sound and when the engine is revved it doesn't get louder. But it's very hard to hear with the engine revving because there is an exhaust leak (needs a new gasket).

These next few points could simply be the noise being transmitted to the cab, but here are a few more observations:

• Sometimes, the noise becomes much more pronounced when the steering wheel is pulled right or left
• I've noticed at least once that the noise was much more pronounced inside the cab with the clutch pedal engaged (or was it when it was disengaged, I don't remember)

I checked the clutch/flywheel bolts, they where all snug. It might just be a normal noise on an old truck and a newbie like myself is just paranoid. But if it is the early stages of rod knock, I will want to sell this truck before it gets any worse...

Any idea what it could be? What should I check??

Thanks!

 

[PrairieDrifter]

It could be early rod knock but it might not be, if it is I wouldn't be that worried if it's a a nice vehicle, or new motor? Or you can find a nice running motor for pretty cheap, that's just what I would do then rebuild the motor you pull out and rebuild it and hop it up some.
That's just me haha

You just have to pull the pan off and install new rod bearings if that's the problem.

 

[79K10]

i had a bad water pump that made a clunking noise kinda how you described, try pulling the belt off the water pump for a min, then start it and see if you still hear it...


or maybe its a bad throw out bearing, same clunking type of noise ... i had to back my sm465 away from the motor to change it, then the noise was gone... .02

 

[MannyDantyla]

I've been driving around today and yesterday and I feel like it's getting more frequent (pretty much every time) and louder. Definitely a clunking sound. What about the output shaft bearing? I'm having a friend come over to listen to it, he's built many race engines.

It could be early rod knock but it might not be, if it is I wouldn't be that worried if it's a a nice vehicle, or new motor? Or you can find a nice running motor for pretty cheap, that's just what I would do then rebuild the motor you pull out and rebuild it and hop it up some.
That's just me haha

You just have to pull the pan off and install new rod bearings if that's the problem.

This might be a foolish question, but can I remove the oil pain and access the crank from underneath the truck with the engine not removed? Seems there's enough room under there.

And I've learned that I can detect if it's rod knock by pulling spark plugs, and also pushing on the piston with a screw driver through the spark plug whole, when the piston is in it's down stroke, and feeling for wiggle.

i had a bad water pump that made a clunking noise kinda how you described, try pulling the belt off the water pump for a min, then start it and see if you still hear it...

or maybe its a bad throw out bearing, same clunking type of noise ... i had to back my sm465 away from the motor to change it, then the noise was gone... .02

Hmm, I'll definitely test that one. And I'm worried about the throw out bearing, I don't want to have to fix that

 

[chengny]

This might be a foolish question, but can I remove the oil pain and access the crank from underneath the truck with the engine not removed? Seems there's enough room under there.

Yes. It will look something like this:

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[MannyDantyla]

I now believe it is loose steering linkage that is getting vibrated by the engine. Nothing I should worry about I guess. Just normal "truck noise". I haven't had this truck for very long, still learning a lot.

If it turns out to be the through-out bearing or output shaft or a rod bearing, I'll post an update.

 

[Lastwords]

One trick I've learned to narrow down where a sound is coming from is a broom handle. Use it like a stethoscope. Put one end to where you think the sound is coming from and put your ear to the other. Move the end around the motor till you can hear it clearly. Being careful of the pullies.