Knocking upon start up..

Good day, I have a 2000 yukon xl that I baby.. always parked in the garage and recently upon starting up and putting it in forward I notice a knocking noise from the engine like valves!!!. Could this be a sensor. The vehicle only as 80km and always treated with synthetic oil. Notice that it is getting worse specially after being parked for a long time. Let me know.

Does it go away after it warms up? What engine does it have?

yes it does and the engine is 5.3 litres. To describe it more.... I start it up in the garage back it up to street 50 feet. Put it in drive to take off when its starts to knock pretty excessively to the point that I am wondering if there is oil in there...... it stops completely when the engine is warmed up....oil levels are o.k... could it be a sensor..

My 2001 does the exact same thing. I wonder if it's what they call a "piston slap".

It's a known problem that GM won't acknowledge as a problem. My '99 started it at 60k miles. A strong dose of Chevron Techron would stop it for a few months. It was none worse at 95k when I sold it. Supposedly, it is a buildup of carbon in combination with the extremely short skirted pistons in the later model engines.

The first time I heard it, I would have bet the farm that it was collapsing lifters.

I have the same exact problem with my '99 Yukon. It's going in the shop tomorrow. I'm being told that GM has had problems with the intakes and anti-freeze may be leaking into the oil breaking it down. I also use synthetic oil and my mechanic commented that I may have saved the engine by doing so. I'll let you know what I find out.

My dad's '00 Silverado did the same thing. He was told through the dealer that these engines have hydraulic lifters and after sitting for a little while they just take some time to "pump up". As long as you let it pump up before driving it would be ok.

Well it looks like I have a major problem. After pulling off the valve cover we saw a significant amount of milky residue. I'm getting major amounts of anti-freeze in the oil. Now it's just a matter of finding out where it's coming from.
I'm gonna start by flushing the system with kerosene and changing the oil to see if it goes away temperarily.
Next, I'll change the intake manifold gasket. I'm replacing the lifters, thermostat, serpentine belt, and water pump at the same time - might as well while in there. I think I may have a collasped lifter or it's slow coming back.
Cam seems OK so far.

I'm hoping it's not the head gasket. I'll find out next week.

So far it looks like a $600-$700 bill with parts and labor.

Can anyone recommend a better way to flush the oil vs. kerosene?

More to follow.....

Why kerosene? Changing the oil should suffice but if you just want peace of mind, change oil twice.

The white goo (Oil/Anti-freeze mixture) was sticking to everything and I was told by my mechanic to flush the system.
I also use synthetic oil and at $25-$30 it seemed more sensible to use an additive. I ended up using this stuff called CRC - High Mileage Engine Flush. It did a decent job getting this stuff out.
The noise is still there, but seemed to go away quicker leading me to believe a have a sticking lifter.
It's going in the shop on Wednesday.

Well, it was definitely the lifters. I had a total of three that were collapsed. Two weren't too bad, but one was shot. I replaced them all and the car is running better than new. The cam was fine.