Engine making a clicking noise

I have a 95 GMC k1500 with a 5.7 L and I can hear a steady clicking sound. It is hardly noticeable when the truck is cold but after about a 30 min trip
I can definitely hear it from in the cab and is very predominate when I listen by the intake. The truck also seems to be lacking power until I get it really warmed up. With a 500 lb load in the box I have a hard time to keep the truck in overdrive while driving at 110 km/hr. Once warmed (30 mins or so driving time) I have no problem keeping it in overdrive. We thought it might be one of the hydraulic lifters broken and banging away but after pulling the valve covers off and checking for some play on the rocker arms I found none. Any more ideas? I am mostly wondering if this will be harmful to my engine. There is about 150000km on the engine and I run synthetic oil. Thanks
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Is this the same problem that some are refering to as piston slap? Seems unusual to me that it gets louder as the engine warms up.
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No, 95's with the 5.7 were not affected by piston slap. Only the new engines (4.8, 5.3, 6.0) have the slap. Sounds like you have something funky going on.................you sure you're not hearing spark knock?
Have you at least pulled all 8 plugs to see what they tell you?
If the engine sucks when cold and gets better as it warms up that would seem to suggest a non-mechanical (i.e. fuel injection, ignition, etc.) source of your woes.
Without more information I can't even fathom a reasonable guess.
Doc

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I will check out the plugs in the morning. I replaced them along with the dist/wires/o2 sensor about 4 months ago. If you can think of any other information that I can find that may be useful I am all for it. Thanks

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I ran a check for codes stored in the computer and there were none. I was going to check the plugs but the blizzard outside helped me to rethink that idea for today. Now the I can hear the sound more pronounced even when the engine is cold.

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by the way what does spark knock sound like? I am not familiar with that one. I do all my own work on my vehicles and I am not to confident with taking it into any shops nearby (live in very rural Alberta) thanks

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