Sounds like rod knocking

[joeysz06]

I have a 71 gmc with an early 80s 350 and a 3 speed with granny gear. I recently started to get a knocking sound from my truck but only in 2nd and 3rd only when gassing if I let off it stops and if I'm parked in neutral I can free rev with no noise. Any advise on where I can start looking.
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[mrein3]

Spark knock?
Maybe try higher octane or pour octane booster in what you already have.

[joeysz06]

I run 91 just cause I feel it runs better
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[JOJABOY]

timing too far advanced would be my guess. does it sound sort of like valves rattling when you are up to speed (like 50 mph) and trying to cruise along? no knocking/rattling when you are accelerating under load like when you are starting off from a light???

[JOJABOY]

did you just buy the truck or do any work on the motor recently?

[TBONE1964]

Usually a rod will make some noise on a free rev. So it only knocks under a load? It it a deep thump or a tinny tic noise, like a lifter? Possible main bearing knock. They are a little hard to diagnose. It is a deep thump like noise and nothing like a valve train noise.

Have you checked the end play on the crank. If you can get both hands on the balancer and push/pull on it and hear any knock then it has to much end play. If you can't move it back and forth, then it is most likey OK.

Also, you may have broke a gear tooth in the trans which will also cause a deep thump type noise under load.

[joeysz06]

No when I take off in second (first) I don't get any noise. As soon as I shift to second(third) it sounds almost like a knock I have not done any engine work. It idles fine
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[joeysz06]

Originally Posted by JOJABOY

did you just buy the truck or do any work on the motor recently?

I have had it for some time
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[joeysz06]

Originally Posted by TBONE1964

Usually a rod will make some noise on a free rev. So it only knocks under a load? It it a deep thump or a tinny tic noise, like a lifter? Possible main bearing knock. They are a little hard to diagnose. It is a deep thump like noise and nothing like a valve train noise.

Have you checked the end play on the crank. If you can get both hands on the balancer and push/pull on it and hear any knock then it has to much end play. If you can't move it back and forth, then it is most likey OK.

Also, you may have broke a gear tooth in the trans which will also cause a deep thump type noise under load.

Ill try that it almost sounds like a hard tic It's only on a load could it be flywheel?
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[joeysz06]

If its a bearing wouldn't it be thru he whole rpm range ?
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[JOJABOY]

Could be any number of things. (cracked flywheel, drive shaft, anything). But if it's something that just started happening. Meaning you have had the truck and it didn't do it in the past and it's not a supplemental effect from some engine work you've done makes me think that something broke.

I had a buddy the other day that had a timing issue where when he was up to speed it sounded like every valve in his motor was rattling. Turned out it was too much advance on his distributor and when he got to cruise speed and the vacuum advance kicked back in it was causing predetonation.

Try unhooking your vacuum advance on your distributor and plug the line coming out of the carb/intake. Then take it for a drive. You will get a little rough idle and a decreased throttle response. But when you get it up to speed top gear for your granny trans....see if you still hear it. If you do, plug vacuum advance can back up and rule that out.

Super easy check just to see if it is timing curve. If no knocking....then you know where to go look.

[TBONE1964]

Not always. Main knocks are similar to rod knocks and they have a sweet spot where they make more noise.

It is one of those things that are hard to diagnose unless I can actually hear it. Check all of the obvious things like the flywheel like you mentioned. There are many things that it could be and hopefully we can help you narrow it down.

[joeysz06]

Originally Posted by JOJABOY

Could be any number of things. (cracked flywheel, drive shaft, anything). But if it's something that just started happening. Meaning you have had the truck and it didn't do it in the past and it's not a supplemental effect from some engine work you've done makes me think that something broke.

I had a buddy the other day that had a timing issue where when he was up to speed it sounded like every valve in his motor was rattling. Turned out it was too much advance on his distributor and when he got to cruise speed and the vacuum advance kicked back in it was causing predetonation.

Try unhooking your vacuum advance on your distributor and plug the line coming out of the carb/intake. Then take it for a drive. You will get a little rough idle and a decreased throttle response. But when you get it up to speed top gear for your granny trans....see if you still hear it. If you do, plug vacuum advance can back up and rule that out.

Super easy check just to see if it is timing curve. If no knocking....then you know where to go look.

Ok I'll try today
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[Transformula]

It's a manual trans with gears in it. Nobody else mentioned that maybe there may be some chipped teeth on 2nd and 3rd gears. If first is quiet, there may be no engine trouble. You can chip a gear tooth and every time it goes around, it will knock. Sometimes you can feel it in the shifter, sometimes you can't. And it can knock on accelleration, and not on coast, or vice-versa.

[special-K]

Does it knock in neutral? Sounds like your trans.