Got me puzzled to say the least

So I have a problem and I don’t have any idea what it is. Running down the road in gear at any RPM above 1700 I have a rattle, it kinda reminds me of an exhaust rattle. Any RPM below 1700 it sounds like a regular ol 350 Chevy. Now here’s what gets me sitting in the driveway in park or neutral it’s not there. It sounds it’s coming from the right bank. Two guys told me it’s a rod walking back and forth on the wrist pin, and two others told me it’s a valve or two. But what gets me I only does it in gear turning 1700+ RPM, and also only under Trottle, when I drop into second gear and the RPM jumps up to 2500 it doesn’t do it. This makes no sense to me. Hopefully one of y’all have a better idea

Experienced and skilled talent needed to hear it first hand in person.. Anythting else is pure guess work.. But I can tell you this.. It is NOT a rod "walking back and forth on the wrist pin". and probably not "a valve or two". The wrist pin is a very tight press fit on the rod and it floats in the piston, no keepers or buttons needed in the piston to keep the wrist pin in place.. If the pin does come loose in the rod, the first thing that happens is the engine has a cast iron hemorrhage.

If you really want my guess, I think you're hearing detonation/pre-ignition from timing being too far advanced or octane too low for the engine's requirement. To determine this, pull about 3 degrees out of the base timing setting, drive it, and listen for a change in the noise.

RustyPile said:

Experienced and skilled talent needed to hear it first hand in person.. Anythting else is pure guess work.. But I can tell you this.. It is NOT a rod "walking back and forth on the wrist pin". and probably not "a valve or two". The wrist pin is a very tight press fit on the rod and it floats in the piston, no keepers or buttons needed in the piston to keep the wrist pin in place.. If the pin does come loose in the rod, the first thing that happens is the engine has a cast iron hemorrhage.

If you really want my guess, I think you're hearing detonation/pre-ignition from timing being too far advanced or octane too low for the engine's requirement. To determine this, pull about 3 degrees out of the base timing setting, drive it, and listen for a change in the noise.

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I know when it comes to something like this it has to be heard in person and felt, I started out running 87 octane when I first got it but since the early fall I’ve been running 89, I was kinda thinking it could be ignition related, when I first got the truck we timed it and it was at 10 degrees advanced and it seemed happy there (or so I thought, I had both mufflers blown out at the time and thought what I was hearing was just them ratting)

Ignition timing, timing advance curve, fuel octane, and compression ratio all go hand-in-hand.. 89 octane will work with compression ratios below 9:1, if the timing advance curve isn't too aggressive -- Vortec heads are more forgiving is these areas.

Pull all belts, alt, AC & try to duplicate. Shouldn’t have any heat shields that are notorious for rattle at specific rpms. Might have a partially collapsed muffler that would account for it.

Copymutt said:

Pull all belts, alt, AC & try to duplicate. Shouldn’t have any heat shields that are notorious for rattle at specific rpms. Might have a partially collapsed muffler that would account for it.

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It’s got 2 brand new Flowmaster FlowFX mufflers on it, the alt and ps belt were done recently, there’s no heat shields on the truck

RustyPile said:

Ignition timing, timing advance curve, fuel octane, and compression ratio all go hand-in-hand.. 89 octane will work with compression ratios below 9:1, if the timing advance curve isn't too aggressive -- Vortec heads are more forgiving is these areas.

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I don’t know a whole lot about the engine other than it being a GM GoodWrench crate engine with 40,000 miles on it roughly

The original GM GoodWrench engines were simply "service replacement" engines.. Built offshore using new parts, and were very "universal".. Back in the mid '80s, my wife and I ran our small shop with 5 employees. We did a lot of service work for businesses in the oilfield industry.. We replaced a lot of the "hotshot" truck engines with them.. They were, and probably still are, very good engines. Some of the guys even ran them in the stock classes at the local dirt track.

I'm just guessing as to your noise, but I can almost guarantee it's not valves or rattling wrist pins..

10 degrees BTC is a bit much for one of those engines.. More like 6 - 8 would be where I'd start... Do like I suggested and take a few degrees out of your base timing.. Connect the vacuum advance to manifold vacuum source if you haven't already done so.

Do you happen to know the specs on the distributor?? Degrees of mechanical advance and vacuum advance?? Base, mechanical, and vacuum degrees should not total more than about 33 - 35 degrees and be "all in" around 2500 - 2800 RPM.. If it comes any sooner, or more that the above mentioned total, you'll have pre-ignition.. The sound the engine makes when this happens sounds like marbles rattling around in a coffee can.

Too much and/or too long will severely damage the engine.. Broken rings, broken or melted piston tops.. All is serious stuff..

What was the last thing done to the truck prior to the noise appearing?

Copymutt said:

What was the last thing done to the truck prior to the noise appearing?

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Only thing I would’ve done prior is new plugs and wires

Kowboy said:

Only thing I would’ve done prior is new plugs and wires

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In doing so, did you check or change the base timing??

RustyPile said:

In doing so, did you check or change the base timing??

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All I did was check base timing and it was at 10 degree

Does the vacuum advance work as it should?

Kowboy said:

So I have a problem and I don’t have any idea what it is. Running down the road in gear at any RPM above 1700 I have a rattle, it kinda reminds me of an exhaust rattle. Any RPM below 1700 it sounds like a regular ol 350 Chevy. Now here’s what gets me sitting in the driveway in park or neutral it’s not there. It sounds it’s coming from the right bank. Two guys told me it’s a rod walking back and forth on the wrist pin, and two others told me it’s a valve or two. But what gets me I only does it in gear turning 1700+ RPM, and also only under Trottle, when I drop into second gear and the RPM jumps up to 2500 it doesn’t do it. This makes no sense to me. Hopefully one of y’all have a better idea

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Maybe the early evap flapper on the right side exhaust rattling.My old van did that,it was a 75 model 350 tho.It was where the pipe connects to the manifold on mine

Kowboy said:

All I did was check base timing and it was at 10 degree

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Ok.. Have you moved the timing back as I previously suggested?? If so, what was the result?? Have you checked the timing in each of the 3 "categories" I mentioned?

Others on here have made thoughtful suggestions. Have you looked into any of them??