cracked flexplate can act like that

 

Been away for a few weeks, but now I'm back on this.

A friend who worked as a GM tech for over 30 years also says it could be a crack or cracks in the flexplate where it bolts to the crankshaft that I wouldn't be able to see from below. I plan on pulling the trans next week to see what I can see.

If no cracks, is there any reason I can't start the engine with flexplate attached? If the noise is still present that would eliminate the converter and trans.

 

A tiny piece of dirt in a lifter will do that.

 

Broken valve spring. One coil broke , enough pressure to close the valve at low rpm ...?

 

Valve springs look and feel OK.
Will inspect flexplate as well as I can.
Intermittently stuck lifter would cause a rocker arm to clatter, but it doesn't seem to be that kind of noise.

 

Sure you can run it like that but without any load on the flexplate you’ll likely not hear or see anything.

 

Is there any possibility it’s from the exhaust manifold or pipes? Clearances change as the exhaust system heats up, so noise and vibration also changes quite a bit sometimes.

 

Joe beat me to it...I was going to say a baffle came loose in your muffler. Sometimes you can tap a flexplate with a dead blow hammer and get a weird sound if its cracked.

 

I already confirmed the trans-to-engine bolts are tight, and I did smack the flexplate with a mallet. Nothing sounded strange. But I will disconnect the converter from the flexplate and re-check it.

Your ideas about a loose baffle in one of the mufflers is worth checking into. In fact, one of the ways I would describe the sound is like a bolt clattering around in an exhaust pipe! The only problem is two friends and I think the noise is coming from the front half of the truck.

When my son had the truck painted, the shop used an outside muffler shop to replace the exhaust system (which I thought was perfectly fine). I think the mufflers are Magnaflow but I'll have to confirm that. Unfortunately they are welded on both ends, so it will be a PITA to check the insides.

I should also take a look at the exhaust manifolds as suggested above. I had planned to remove and clean them up anyway.

Thanks for all the ideas. I WILL get this thing beat!

 

When my son had the truck painted, the shop used an outside muffler shop to replace the exhaust system (which I thought was perfectly fine). I think the mufflers are Magnaflow but I'll have to confirm that. Unfortunately they are welded on both ends, so it will be a PITA to check the insides.

When the baffle in my muffler came loose, it was easy to tell by tapping on the muffler.

 

JOE BEAT ME AGAIN!

So...the class has learned to diagnose problems with a hammer LOL

Have you tried a mechanics stethoscope or even the patented giant screwdriver to listen for the noise?



Does anyone know what a loose fin in a converter sounds like? I'd also listen to the waterpump to rule it out.

 

You might think I had this happen to me recently...

 

To be honest, with the engine off, I just rapped the muffler with my fist and I could hear the loose baffle banging.

 

Spent some more time today trying to isolate the noise. Two of us agree it's coming from the area of the engine/trans junction, so I'm hoping it's the flexplate. We'll pull the TH350 next week to take a look. Kind of strange to me, however, that we even hear the noise in Park, with little to no load on the converter.

In the department of "it never ends", my fairly new AGR power steering pump is making lots of of noise externally, and it sounds really nasty when I use a stethoscope to listen to it. What pisses me off is the pump it replaced was one I got at a junkyard around 1990 and it still worked fine. Recently bought an AGR steering box along with a new pump from them because it seemed like the way to go. They're good folks though, and only 45 minutes away.

Was thinking about replacing the >20 year-old full size starter, but I'm just gonna stick with it.

 

OK, guys. You're never gonna believe this. I had removed the trans dust cover to listen and watch underneath the truck while my buddy revved the engine. No matter what we did, we could not hear the noise. We tried for 5 minutes to reproduce it, but could not.

A few minutes later my buddy pulled the dust cover out from under the truck, and immediately noticed some damage on it. Turns out it was in the same place where the converter bolts go flying by! So the bolt heads were spinning past the plastic cover, sometimes hitting it, sometimes not. I'm not gonna try to determine why it didn't make noise all the the time, I'm just happy that it's gone!

So, the problem was related to the flexplate, but in a good way.

I had installed that cover around 20 years ago. For all I know it could have been hitting the flexplate from day one, eventually warping it backwards causing the noise.

 

Thats a new one!. Glad it's a done deal know but the longer we do this, we never stop seeing it all.

 

Yes, that's a new one, for sure!

One more thing. What brand dust covers fit well?

Ah, looking at Summit Racing, I see where most have bad reviews. But it looks like the Proform plastic and OER chrome models have decent reviews. I think I'll go with steel this time.

 

how about hit a junk yard and get a GM unit,

 

That's some good advice. Haven't been to a junkyard in years. The one near my last house turned into a subdivision! Haven't even looked to see what's near me now.

 

This was one of the symptoms I mentioned above: "Under heavy acceleration, when the noise starts the exhaust note changes. It goes from a rumble to more like a rat-tat-tat glass pack."

Well, it's totally gone! Probably was just the dust cover getting smacked in a different manner, or a different place as RPMs changed. Pretty crazy!

Have to say that the noise had been around for quite some time to a much lesser extent. It initially sounded like something rattling around under the bed or cab, mostly when driving by a car or bridge railing that would echo the sound back to me.

I am super happy that the noises are gone! Almost forgot how well the 18 year old, budget-built 355 runs and sounds!