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  1. #1

    Default Slap/Knock of some sort. Engine/Tranny?

    Okay, for now, I'm not sure where this knocking/slapping is coming from. Sorry, I'll go look tomorrow.

    I've driven with this problem for a while now and it seems to come and go whenevr it pleases. In the morning when I start up,and its in park, it'll make the noise in a constant pattern 3 times every second lets say. After the truck warms up and drops to around 600rmps, it'll stop. Once I drive off, it happens in the 1500-2000 rpm range and after that nothing. I just drove it around for 5 minutes and when i parked at home, no knock. Sometimes after driving a while, and I come to a stop at a red light, and it's in drive and it'll knock constantly and wont stop unless i build up the rpms after 2000rpm. It doesn't SEEM to be coming from the engine, BUT I may be wrong. It seems to be coming from the transmission.

    Any help will be greatly appreciated and I'll have more info in a short time. Thanks.

  2. #2
    Legend unplugged's Avatar
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    Sep 2006
    High Deserts of SoCal


    Help us help you. Please remember to include model/year/engine/trans/mods, etc.

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    Piston slap is common in the GM V8's, but not harmful. Detonation is a knocking sound that is pre-ignition of the fuel/air charge and is bad for the engine. I don't know where to start with tranny sounds. Maybe one of the other members will post some suggestions.

    In the meantime do a search on piston slap and detonation. Youtube has several videos with the sound.

    No real fix is necessary for piston slap. If you let the truck warm up before you drive, it should go away. For detonation, I would look into a tune up and maybe run a can of SeaFoam to clear carbon deposits from the combustion chambers.
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  3. #3


    [ame=""]YouTube - Knocking/Piston Slap Noise in Denali 6.0L Engine[/ame]

    This seems to be identical to what I have but in mine, the time intervals are shorter and quicker.

    I turn on the car and let it warm up before I go to school. As I said, it appears to happen when it wants to or after driving for, lets say, more than 30 minutes.

  4. #4


    Okay, so this is from my phone so it's really crappy quality and sorry I was moving it around so much but you can hear the slap. After looking under the car, it sounded like it was coming from between the tranny and engine. (I could just be crazy but this is the video.)

    Oh and I started up this morning, no slap. I started vacuuming and 10 minutes later it starts to slap out of no where.

    I'm going to use 91 (maybe 93 if I find it) octane today and see what happens. Could it make a difference?

    [ame=""]YouTube - Slap[/ame]
    Last edited by plmichal; 10-31-2009 at 11:36 AM.

  5. #5
    Legend unplugged's Avatar
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    Sep 2006
    High Deserts of SoCal


    As I mentioned in my previous post, piston slap is not harmful. You can fix it, but tearing the motor apart is usually more than most people want to do. Most people just put up with the noise.

    Detonation/pre-ignition is caused by many things, and should be looked at.
    Detonation usually occurs when the engine is under load, not when it is at idle.
    You may have a faulty knock sensor. The knock sensor detects detonation and adjusts the engine timing.
    Higher octane fuel may help. Have you tried the SeaFoam?

  6. #6


    How is metal against metal not harmful?

    Well I don't think it detonation because it'll slap at idle and after 2000rpm or so it just goes away.

    I'll check the knock sensor and I'll use seafoam sometime soon. I'll let you know what happens. Thanks.

  7. #7


    I've got this exact same problem in my 2009 with the 5.3L active fuel management engine. It has less than 150 miles on it and sounds like its a strange exhaust note that I thought was from the engine dropping to 4 cylinders after a short idle period. I'm going to the dealership this week to see what their take is on it and I'll let you know.

  8. #8


    that sounds almost like a rocker arm loose or a collapse lifter.the sound is more pronounced than the regular chevy piston slap issue.
    Last edited by Papi Chino; 11-03-2009 at 12:02 PM.
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  9. #9


    The sounds seems to be coming from the bottom of the car. In the video (and when I was under the truck) it sounded almost to be somewhere in between the tranny and engine... if it was a rocker, wouldn't it be louder on top of the engine? In the video I showed how you can barely hear it over the engine bay.

  10. #10


    My 03 started making a metallic knocking noise last week... sounds horrible, so bad I'm embarrassed to drive it around as everyone turns and looks when I drive by. Hard to tell with your video but it could be the same problem I have which ended up being my catalytic converter had come loose inside the converter housing.

    With truck running (parking brake on) crawl under and listen to the converters, the knock I have clearly comes from the converter and not the tranny or engine like I had originally thought.

    my fix, eBay... found a nice set of stainless headers complete with new high flow cats and Y pipe, with O2 bungs welded in all the right spots.... Don't have them on yet but once I do I will post on here.... good luck to you

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