Looking at a 91 dually 2wd tow rig with a big block. Has a noise that sounds like a loose torque converter flex plate noise but is up top and sounds sharper than a pushrod banging the cover. Valve covers have no dents in them and the engine runs smoothly. Just has the noise. It runs decent oil pressure at idle 35/40 and it drops to 30 when you put it in gear and oscillates with the engine rotation.
Is it possible this thing may have spun a cam bearing? I can buy this truck cheap as hell, but don't want to redo the entire engine right now.
For all you big block guys in Pirate land, what would sound this loud?
I have heard rat motors with busted pushrods before, this is more metallic, like something is not getting lubed.
I also looked at the vibration dampener, it was not wobbling or giving me any indication the rubber had slipped. This is not a rod knock or piston slap.
I did not have any diagnostic stuff with me, was going to go back with a timing light and compression tester.
Has the gas intake with the adapter for the throttle body, kinda of odd looking. I know its factory, just surprised GM did not cast up a proper manifold for it.
Sorry for the length of this thread just trying to let you know what I have looked at to this point. Any and all diagnostic suggestions welcome!

I have heard cracked flex plate make that noise.
if plate is verified good, then I'd pull valve covers.

loose converter bolts
ring gear busting off and clipping the nose of the starter
cracked flex plate
the crank locating pin may have worked out and be clipping the block.

loose spark plug in the head
burned head gasket between the cylinders
loose dist cap being cliped by the rotor

Try pulling the plugs out and turn it over by hand....see if it hangs up or you can hear it better.

I have the same noise in my '89 1 ton dually. I've been running mine for awhile this way and it's still ok. But I'm pulling the motor(swapping in a diesel) and I'm checking the flexplate then. The noise is irritating!

loose converter bolts
ring gear busting off and clipping the nose of the starter
cracked flex plate
the crank locating pin may have worked out and be clipping the block.

loose spark plug in the head
burned head gasket between the cylinders
loose dist cap being cliped by the rotor

X2, most likely a cheap and easy fix. The oil pressure is a good indicator that the bearings are still good so i would suspect something else. Loose converter bolts will make a knocking sound as well is a cracked flex plate but i have heard cracked flex plates squeak too. Check for the bell housing bolts to be tight. If you say the truck is cheap then i would buy it and worry about the noise after you get it home.

I've dealt with cracked flex plates and pushrod issues with big blocks in the past. This is a sharper cracking more metallic sound.
While engine was running I bent down and looked at the torque converter and flex plate (the cover was allready off) and it was running smooth with no wobble and the noise was less as I bent down under the truck. Pretty sure it is top end noise. But it is the nastiest lifter/pushrod noise I've ever heard.
I thought maybe vibration dampener but it showed no signs of damage either.
I am going back to check it out with a compression tester and pull the valve covers.
The oil looks good and the cooling system has got fresh coolant and is nice and clean.
There are indications someone has been chasing this problem and does not want to fix it.
I'll go back and read the plugs, compression test it and pull the valve covers.
This thing sounds nasty, like a collapsed lifter just hammering away at the pushrod, it is fricking loud.
If purchased this is gonna be a tow rig/work truck. It is a 91 extracab 2wd. Looks to be a Dana 70 rear axle and has 4.10 gears with a turbo 400. I'm thinking some type of RV cam and lifters if needed. We built a 468 years ago that fell on its face at 4000 rpm, but pulled like a freight train up to that. That engine acted almost like a diesel. I think it was a Sig Erson RV cam. Cam technology has changed a lot since then, any suggestions?
Thanks for all the replies, I appreciate the help and suggestions.

Hopefully not piston slap from a collapsed skirt from detonation. They don't need to collapse much to rap like crazy.

Yea hope your right . A piston sounding this bad usually won't run as good as this does before it scores the cylinder and starts self destuctiong. Sound does not appear to be in the block. The removal of the valve covers and plug read will tell the story...I hope!:)

P.S. If I can get it cheap enough, I'll just redo the motor. Just hoping to avoid that for now.

Summer boat season is coming and I want a cheap tow rig.

Chasing noise on a V8 can be frustrating at best because it can be in the back and sound like the front-or be down low but sound like the top. Also I would gank spark plugs wires one at a time while it is running to see if that changes the character of the noise at all. If it does yer on yer way. I have seen broke crankshafts make weird noises with no in/out movements on either end depending on where the thrust bearing is located and how the break is made. Will be interesting to hear back from you on what it is if you can diagnose it. Another thing would be to gank the oil filter and cut off the can just above the flange. Inspect the accordian webs for any metal fragments. Hopefully the noise is not from a way stretched timing chain slapping on the cover getting ready to shake hands with you-sorta speak.

Thought of that too, no leaks of oil, so the chain has not wore through the cover yet.:D
Thats what is making this truck so intriguing to me. The noise is different than any others I've heard from ailing motors and my curiousity is the motivating factor here. Most bottom end sounds are deeper and this is real metallicy. I may be fooled into thinking this is a cheap fix. What has got my interest is it seems to idle smoothly, rev up fine and as the engine warms the noise diminishes a little, very little though.
If I can buy the truck cheap enough, I'll figure it out and post up the results.
I'm gonna try to look at it again this week.

look at the exhaust manifolds..

sometimes a hole in one can sound very metallic.

Ok, thanks. I did look for possible cracks or exhaust discoloration. It also has screw in plugs were smog tubes would go into the manifolds. Those are not leaking for sure. Not sure if it originally had a pump or not, no bare brackets for the pump mount. Has the adapter on the gas manifold for the TBI. Kind of a bastardized setup. Gonna have to learn more about the original setup and maybe use that as bargaining chip to lower price offer for non-compliance of our Kalyfornicate you smog laws!:D Maybe get him to pay me to take it!:laughing:

For exhaust manifold leaks-that are tough to define where-I use some ATF and introduce it into a running engine-either down the throat or a vacuum access in the carb or throttle body ( I use a pistol grip oil can with a short jumper of vacuum line for fittings). If it starts smoking out the tail pipe and not anywhere under the hood, it should rule out a gap or hole in the exhaust manifolds. Use a good light. If you have any Seafoam around that will make it smoke betterer.:grinpimp:

When I went to look at it was in his back yard. I had to dodge his dogs land mines, kinda of a pain to really get a good look at it. I did OK for the circumstances but when I go back I'll have him move it out of there so I am not crawling in the dirt and dodging dog poop.

I take it the owner/seller has no idea what the noise is? Point is if the truck is drivable-why would he sell it unless it embaresses him?:confused:

He thinks its valve noise, but is unwilling to go much further. I suspect he checked the converter and changed the coolant and maybe the oil and then gave up. He has got a back yard full of Fords (no shit about 6 Ford pickups) and this is the only Chevy. I think he is a Ford guy and just does not want to fawk with it. Or he knows more than he is letting on and wants to dump it!

didnt read the whole post but have you checked your serpentine belt tensioner i had one sound like a rod knocking but was coming from top of engine took forever to find pullled intake ,cam all lifters checked valves the putting it all back together (not thinking i found the problem) i dropped the damn tensioner on the ground and a chill went through my body as i heard my noise replaced it and never heard it again





might take a stethoscope and try to listen all over the block and heads and intake for your noise

I'd laugh my ass off if it was a belt tensioner! I'm taking a stethoscope, timing light, plug wrench and compression tester when I go back.

the funky adapter is a factory piece. they chose to use an adapter instead of building a specific manifold for the 454 at the time, probably because they had plenty of intakes lying around.

Yes, I remember reading about that, (maybe you mentioned it someplace). funny thing is I found another one of these trucks too. On my way to my in-laws the other day there is another one setting in a field.