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Odd Sound from 2013 GMC Sierra 1500 SL 4.8

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So back in November I was driving around when I noticed that there was a knocking/tapping sound coming from under the truck or in the engine bay. I couldn't tell. I took it to a local shop and they said it wasn't the transmission. 


The sound later stopped sounding so metallic and now just sounds like plastic hitting plastic or something (it's not lol). It starts on crank up, speeds up as I accelerate but doesn't sound "harder/worse" if you will upon acceleration. When I get up to about 60mph and push to 70mph it acts like it doesn't want to shift, I have to let off the pedal and it will shift, then I can continue the push for 70-75. 


I also noticed that my transmission pan now has what looks to be oil residue on it. Oil, not transmission. I did not have an oil leak prior and the underside was always clean until now.

I also started getting a cylinder 8 misfire as soon as this started, so here is what I have done so far:

Coil Packs

Ignition Wires

Spark Plugs

Checked fluids (still same level each time)

Tried fuel injector cleaner with 93 Gasoline, finished that tank and went back to 85 and the CE light went off, and came back the next day and will go off every now and then for a few hours and then come back on. 


I honestly have no clue what this is or could be. I've been told the starter but isn't that, torque converter, piston slap, etc. I don't know where to look so I am hoping someone else has had this issue before. Thanks in advance. 

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19 hours ago, richard wysong said:

The sound may be a cracked flexplate or lifter failure ,the latter would explain the misfire. The oil leak may be the rear main seal

Rear main seems to be fine, not sure where exactly it is but I have had that leak on another vehicle and know it isn't that. 

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