when i turn on my dads truck, there is a loud knockin noise, after a while it goes away.... what could it be ??? its a 2000 silverado with a 5.3, 77K miles on it.

 

It's something GM knows about and never did anything about. It's a cold start knock. GM won't fix it and tons of people have it. It's not really anything to worry about though.

 

+1, u can tell them about it and they'll tell u its a piston slap and goes away when the rings heat up. Supposedly non harmful

 

The famous tic. Don't sweat it, most of em do it, some more then others..

John

 

oh ok... thanks for the input. it didnt do it before thats why i was kinda freaked out. i thought i had pushed the gas a lil too hard, lol, since my dad drives it like a 4 cyl... and wasnt used to the abuse i give mine !!! haha. thanks

 

Mine (2004 5.3 Silverado) just did this for the first time on Monday morning. I'm familiar with piston slap from having owned 3 genIII small blocks. This sounded like a lifter not pumping up until the oil got warm. It did the same thing on Tuesday morning but I shut it off and drove another car.
I'm switching to a lighter weight oil to see if that helps. It has been colder than normal here this past weekend. I'll post the results of my thinner oil experiment.

BTW thinner = 5W30 instead of 10W30

 

yeah... that might help. i mean, its winter and all but here in houston it doesnt get past 30 degrees.... but anyways, hope it fixes the problem.

 

mine just started doing it a few weeks ago. i got a 02 with 68000 on the clock

 

I highly recomend checking the oil filter when your engine starts knocking or tapping and you notice any loss of pressure at all. As it turns out, at my last oil change, Advance was running a special. I bought 5 Qts of Castrol and a Fram filter for $13. I always use an AC Delco filter and Amsoil or Pennzoil. But the oil was a bit dark so I planned on doing a short cycle oil change to help clean it up. Of course I forgot all about it. My Castrol had sludged up and clogged the filter, thus the 20/40 oil pressure instead of the usual 40/60. There was thick oil on top of the filter. Not quite what I would consider "sludge" yet but still nasty.

I added another filter with a quart of Marvel Mystery Oil to remove any deposits that might still be present. Drove it for a day. Drained it while the engine was hot. Replaced it with Pennzoil / AC Delco and a quart of Lucas. It hasn't made a peep since I added the Marvel to the oil and pressure is back up to 40/60.

Also, I was having an unexplained loss of coolant since November, having to add ~ 1/4 gallon per month. I haven't pulled the valve covers to see if my heads were made by Castech (known to crack) but I added the GM coolant tablets when I changed the oil. My coolant hasn't dropped and my oil is still clean now. So maybe the sludge was being caused by moisture in the oil. That's the grand diagnosis that I was able to come up with...

 

that could be it... because my dad or i change the oil in his truck, he always uses pennzoil, my dad was out of town and my mom kept buggin me to change the oil. although no one was usin the damn truck, anyways, i didnt change it and she took it to walmart to get the oil changed. i think they put the bulk oil crap in there, so that is what might be causin the knockin noise. i will replace the oil and post the results.