I posted this over on the IFS board and I thought I'd throw it out here to see if anyone could help. I have a 98 Vortec 5.7L 350. I recently went on vacation and the truck sat at home for about 6 days. When I started it up to go to work yesterday it had a loud tapping sound like a valve maybe. I stopped after a little driving. Everytime I started it after it sat for a good little while it would do this. This morning it started with no sounds. It is pretty warm out here so I wouldn't think it would be because it was cold. Any ideas?
It does have cheap gas in it right now. It sounds more like a tapping of a valve than an engine knocking from gas. Someone mentioned it might me a sticky valve due to carbon buildup and that it happens with Vortecs and wasn't something to worry about, but I don't know. They said use some GM top engine cleaner for it. I doesn't happen all the time and goes away so I don't think it is anything real major but I have been wrong before...
I was thinking that as well since it didn't do it this morning. I'll see what it does when I leave for lunch. I'll put better gas in it tonight probably and I'll check into the cleaner when I do my tune-up.
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<font color=green>This happened to my brother-in-law on his 98 Burb. We used flushing oil on it twice and a can of Marvel in the oil, the tapping got better immeadiately and gradually went away completely over about a week.