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  1. #1

    Default 1995 Suburban Knocking ?????

    Ive got a 95 1500 5.7 4x4 suburban LT

    Ive got a strange knocking that that i cant pinpoint and looking for some ideas.
    I AM A MECHANIC so im not a dum dum but this has me stumped.

    Ok in the morning when its first started my motor has a light knock (not like a spun rod bearing but a knocking none the less. its not noticeable inside but outside it is if you listen for it. The knock is only at idle andwhen cold. once its warmed up it goes away ALOT.

    Ive gotten under the truck with a screw driver to listen with to try and pinpoint the knock and i can not find it in the motor. I know its in the general area of the passanger side rear of the motor. for litening to the motor without the screwdriver.

    the only thing ive noticed beyond that is that the exhaust does not have much pressure coming out the tail pipe and i dont have any leaks. Could a clogge dcat cause the knocking ?
    any other ideas?

  2. #2


    Try an oil additive to see if it quiets down. Pull the dipstick out and see if it stops. I've heard of bent dipsticks that the crank will hit and make a knocking noise. When you first start it, when everything is cold, crawl under it and put your foot on the exhaust pipe and push. If the sound changes or the knock goes away, find what is loose and fix it. The noise is coming from the bottom of the engine, right? Listen to the top end too. What is the oil pressure? Low oil pressure will cause a knock. Low oil pressure would indicate worn bearings or a worn oil pump. It could be a worn thrust bearing on the crankshaft also, but that is usually a heavy knock right at startup. How is the power? A clogged cat robs the power and it will barely run. Also check the motor mounts and tranny mount to see if one is loose or broken. Check the starter to see if it is mounted tight. Just as a guess, I would say its worn rod bearings, last journal. I hope these ideas help you, good luck!

  3. #3
    stephan's Avatar
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    Sep 2010
    Far West, Oregon USA,
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    Welcome to the GM truck club Colin.
    Is this noise at the top of the engine? The fact that it goes away when the engine warms up sounds suspicously like the GM piston knock/slap. There are several threads on this site along with several of the GM corporate technical bulletins about this. Put "piston knock" or "piston slap" into the search box at the top of this page & you can read about it & decide if this is your knock.
    1988 Chevy C-3500 2wd (no pic)
    350 c.i. 5.7 L Stock Block, 4 Bolt Mains
    L-31 Vortec Heads, Edelbrock Cam & Intake,
    Holley 650, Flowtech Headers, Magnaflow exh.
    Jet Trans 700R4, B&M Ratchet, 4:10 gears,
    3" susp. lift kit "shadetree"
    No rev limiter, No speed limiter lol

  4. #4


    thats the thing, i have great oil pressure almost 40psi when at idle warmed up. And it runs great. its also not a fast knock abotu 1 or 2 knocks per second and its not a double knock. its not in the top end, but not the bottom either however its much more audible underneath. it sounds like some type of rotating knock though. As for piston slap id does sound similar to the videos ive seen but i though it was only on the vortec motors

    ---------- Post added at 10:29 AM ---------- Previous post was at 08:31 AM ----------

    i made a video of it, is the link

    i went up to the machine shop that does my rebuilds and he said it sounds like piston slap, take note that it is not as loud as it sounds in the video

  5. #5


    Just wondering if you found out what was causing the knock... have the same thing going on with my '99 K1500 5.7l. It just started last week and driving me nuts trying to find out what is causing it. I have a random P0430 code, so I'm hoping it is the cat on the passenger's side.
    Mark - Arizona

    '99 GMC Suburban K1500

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