Oil light flashes on and off

it is possibly piston slap,which is normal,some car company like GM also use slightly bigger gap between piston and cylinder on (3.1,3.4,3.8 liter engines) to help engine last longer,in return you get some piston slap when ENG cold,if it goes away after it warms up don't worry,it is normal

sounds like a crushed valve lifter

Well it could be the sending unit for the gauge.
But odds are the oil pump or engine bearings are worn out.
As the engine warms up, the metal spaces expand and the oil gets thinner. The oil pressure drops some under normal conditions, but not more than 10 or 15psi. Normal idle pressure with the engine warmed up is probably 25psi.
You could check oil pressure with a mechanical gauge to make sure its not the sending unit.
Once the oil pressure starts staying below 20psi for long periods you'll start hearing noises.

Accelleration noises of the magnitude you descibe makes me think the engines crank shaft bearings have gone. Has the oil pressure light come on recently and what's the oil level like.
Without being there to hear it, it's very difficult to diagnose. You should detect noises and rattle if you rev it when in neutral if it is the engine. If the engine is smooth in neutral and the oil is good you may have a bad diff or cv joint

this does sound like theres a loose line tapping the engine/frame,during warm up the engine idles faster which could make it vibrate faster causing the the clanking or tapping noise,if this has started after trans fluid change,then check the lines running from the trani to radiator sometimes the service person might bang into something causing plastic clips that hold the lines to break,try getting under it and have someone start the truck and listen to see where it's coming from also hold the lines & see if the noise stops,the service guy could have unbolted something & never put the bolt back causing the clank ex.line may have a clip holding it thats bolted to the trani pan ?good luck

yes chevys 3.4 and 3.5 have common lifter noise, have you checked the oil level. I would a better oil ,and I put lucas oil treat in my customers cars after oil changes

That's really bad ... First of all check oil level when its cold... The clanking sounds are your valves running dry. This will cause massive damage the heads. If you oil is low fill it back up & check the engine for leaks & hope there is not valve damage. If the oil was low and you filled it & you still get the clanking noise the valves are permanently damaged & the head/heads & valves will need to be replaced.. Big costs

hi.
a transmission problem when hot, but ok when cold, would suggest that the oil in the gearbox is getting too thin when it heats up.

you say that the transmission has been pulled out but do you know if the oil was actually changed in it.
if the oil has not been changed, drain it and replace with the specified oil.
if it was changed recently, then i would consider using a thicker grade transmission oil instead.

changing gearbox oil, is normally a matter of removing the drain plug on the bottom to drain the old oil, and then refilling using the level plug on the side of the engine.

Your oil is thinning out too much. Lucas oil additive would be a great thing for this or an oil change with a little heavier weight oil The other cause of this could be piston slap from low grade fuels.

That is oil not getting in the motor. go with a light high mileage oil my want to use some marvel mystery oil to keep the walls slick.