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Knocking Noise at Start Up

2000 Blazer 4wd, automatic, 122000 miles

In the last couple of weeks, if the Blazer isn't driven for 3 or more days, there is a slight knocking noise for 15 to 20 seconds after start up. Oil level and pressure are fine. I'm guessing it's the lifters, but I'm open to any suggestions.


Ed B.


  • How long since the last new oil filter? Any oil pressure gauge or light info would be helpful.
  • Lifters is a reasonable guess. At 122K, most everybody starts making a little noise. It could be just an accessory, like the water pump, power steering pump, alternator starting to mis-behave. If you have access to an engine stethoscope, you might be able to narrow it down a bit based on what part of the engine the sound is coming from. I'd check all the fluid levels including the automatic transmission and power steering pump fluid just to make sure one of them being low is not the cause.
  • Is it a "tap, tap, tap" kind of noise or is it more like a heavy "thump, thump, thump.."
  • Another question would be: Does it happen with every start? A drained down lifter or two would not happen every time due to the engine's camshaft stopping at various positions. Another possibility is piston slap but that would likely take a little longer to go away as the pistons heat and expand. A little piston slap at startup is harmless as is a little temporary loose valve lifter noise.
  • Four months and 1300 miles since last oil change. Oil level ~3/4 full, oil pressure normal (40 psi cold, 20 psi warmed up at idle).

    All fluid levels good, alternator, ps, and ac quiet. Noise did not occur after sitting overnight. Noise duration is too short to track down.

    Noise is closer to a light tapping, definitely not a heavy thump. Similar to an exhaust manifold leak, but not as loud.

    Noise does not happen every time. More likely to occur after the truck has been sitting a few days.

    Ed B.
  • I'll guess at valve noise. Does it use hydraulic lifters?

    I'd also not actually rule out a very small manifold leak - one that mostly seals up almost immediately with some heat expansion.
  • Lifter almost for sure ... not an issue at this point ... could get worse in another 100k or so. (Also bet it puffs a little smoke out the exhaust when started after overnite or longer)
  • Probably the lifters and not a big deal. You might try a different brand of oil filter. Some don't have a very good 'anti-drainback' valve and the pump has to refill the filter before you get any oil pressure. My 94 used to make a little noise at startup if it sat overnight. I switched to Wix filters and it doesn't anymore.
  • Offhand, it sounds like a valve lifter rattle but the oil pressure also appears to be a bit low to me at 20 PSI on a warm idle. (The lower oil pressure would not cause a lifter rattle. A lifter rattle would be caused by a faulty lifter; either a mechanical fault or grunge which is causing it to bleed off.)
  • I drove it into work today and kept a close eye on the pressure gauge. This is the dash gauge so take the readings with a grain of salt.

    Cold idle at start up - 50 psi
    After 10 minutes driving, idle and cruise - 30/45 psi respectively
    Fully warmed up, idle and cruise (40-45 mph, 1300 rpm) - 20/40 psi

    No rattle at start up after sitting overnight. I'm more curious than concerned about the rattle at this point. It may be time to change from 5w-30 to 10w-30 oil.

    Thanks for all the replies,

    Ed B.
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