Thumping noise coming from intake

[Dirtymaxx73]

Hoping someone can help.. I’ve got a thumping noise coming from my air box that started randomly the other morning. it’s a 01 Silverado 3500 lb7. Did a compression test already and all cylinders were between 310 and 325 3 cranks cold.. it does it warm and cold. Does get faster when rpm’s increase but can’t really hear it after about 25mph. No loss of power that I can tell. No white smoke. No blow by. Took the filter off and still does it. Thought it was something to do with the turbo or piping so I swapped the turbo out with another one I had and checked all the piping for the turbo with no success. Oh and balance rates are all good. No codes either.

Source: https://youtube.com/shorts/deiuW7LE5QA?feature=share

Source: https://youtube.com/shorts/obTKNm50P3w?feature=share

 

[2004LB7]

Welcome

Maybe this: https://www.duramaxdiesels.com/forum/threads/sound-coming-from-airbox.88912/

And this: https://www.duramaxdiesels.com/forum/threads/knock-sound-in-intake.79882/#post-1106112

 

[Delta.Speed]

That first thred posted above is me. One of my trucks, a 2002 LB7, makes that same exact thumping sound from the intake, except mine isn't quite as loud.

But definitely the same sound frequency and tone.

My truck has new heads with screw in cups, pushrods, double springs, billet injector hold downs, 45% SAC injectors from Lincoln Diesel, and a few other goodies and still makes the sound.

No symptoms other than the sound. Balance rates all checking out okay.

After that thread, I chalked it up to engine resonance like some others said cause I had other stuff to worry about with the truck.

I recently acquired a 2004 LB7 that I'm dying to take the intake lid off of to see if it makes the same sound.

Now that someone else has brought it up I'm interested to see what kind of responses you get. It'd be nice to know if it's really normal or if there may be something that needs attention.

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[Dirtymaxx73]

I definitely don’t think it’s normal my last motor never sounded like that and this motor didn’t sound like it does now until randomly it did a week or two ago. Don’t think sounds like that appear for no reason and are normal. Gonna take the valve covers off I think and see if I can find out anything.

 

[2004LB7]

I definitely don’t think it’s normal my last motor never sounded like that and this motor didn’t sound like it does now until randomly it did a week or two ago. Don’t think sounds like that appear for no reason and are normal. Gonna take the valve covers off I think and see if I can find out anything.

Check the torque on the hold downs when your in there. If you find any that are loose it might be a bent hold down or stretched bolt

 

[gmduramax]

Does it have the vacuum pump on it? You sure it isn’t that

 

[Delta.Speed]

Does it have the vacuum pump on it? You sure it isn’t that

My truck does it, and it has no vacuum pump.

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[juddski88]

Sounds like a cross leak from exhaust to intake to me. Every time I've torn down an engine with that going on, it's had either bent pushrods, or bent valve, or leaking seat, or broken rocker/bridge, or all of the above.

 

[Delta.Speed]

Sounds like a cross leak from exhaust to intake to me. Every time I've torn down an engine with that going on, it's had either bent pushrods, or bent valve, or leaking seat, or broken rocker/bridge, or all of the above.

Any recommendations on what could cause that other than the ones you've listed.

I dont mean to hijack the thread, but I was where he is over a year ago. I tore into the motor all the way down to the head gaskets hoping to find something on my way there.

The only thing I found was a failed crush washer and a lifted cup.

No damaged mechanical parts.

And even after going back with chromoly pushrods, new heads, springs, and more. Mine still does it.

No lack of compression either as you'd expect from an exhaust to intake leak.

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[NC-smokinlmm]

I say bent push rod.

 

[Dirtymaxx73]

Well I found it. Left one is the one I just took out. Right is off my parts motor.

 

[Dirtymaxx73]

Exhaust valve on cylinder 1

 

[juddski88]

Well I found it. Left one is the one I just took out. Right is off my parts motor.

How did the bridge look?

 

[Dirtymaxx73]

How did the bridge look?

Surprisingly looked fine. Almost finished with it. Just gotta put the air filter on and start it.

 

[juddski88]

Surprisingly looked fine. Almost finished with it. Just gotta put the air filter on and start it.

You checked the pushrod too? Was the lash adjustment nut tight?

 

[Dirtymaxx73]

Yeah the nut was tight. I re adjusted all the valves. So far so good!! She’s running great again!! Thank you guys.

 

[JoshH]

Was the little wear pad still in place on top of the bridge?

 

[Dirtymaxx73]

Yup was still there and bridge was perfectly fine somehow.

 

[Delta.Speed]

Great thread and sweet find!! I'm gonna tear back into mine and recheck all of the rockers and bridges. Those were the only parts I reused in my top end. My pushrods are all new so I must have something similar going on.

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[Dirtymaxx73]

So my truck started making the same noise again.. took off the driver side valve cover to find that cylinder 6 exhaust valve rocker is missing the piece on the bottom that contacts the bridge.. anyone have any idea on why now two of these pieces have fell off?