Recently (before and after my transmission rebuild), I've noticed a problem with my truck not having the power it should at higher RPM's. The truck will start, idle, and slowly accelerate fine, but once the tach goes above 3000RPM, there will be a light rattle from the engine compartment which increases depending on how hard you're accelerating or how high the RPM's are...following that 3,000RPM mark, the SES light will flash until the truck either shifts or you let off the accelerator. After about 30-60 seconds, the light stops flashing, but may stay on due to a couple other codes. This never happens when parked/in neutral, it only happens under load.
The truck has brand new AC Delco iridium spark plugs/wires, brand new fuel filter, clean air filter, and of course the transmission rebuild with new torque converter. For a while, I've been getting P0300 (random multiple misfire) and P1133 (heated o2 sensor bank 1 sensor one, which IIRC is the rear o2 sensor), but occasionally after these hard acceleration episodes, it would give me a P0303 (Cyl 3 misfire) code, so I tried swapping coil packs between the #3 and #5 cylinders, with no luck, so the coil packs seem to be in operation. Last night, we did do a pressure test on the fuel pump...upon turn-on, the pressure goes up to around 54-55psi, and once the truck is started and idling, the pressure drops to between 48-50psi, which I was told is normal. I've also used MAF cleaner on the MAF sensor, although I wasn't pulling codes. No luck there either.
We're at a loss with this, sort of trying to replace the fuel pump and see if that works...it has been emitting a high pitched whine for a while, but not all the time, so I'm not sure if the pump would be to blame or not. Truck is, of course, a 2001 Silverado 1500 Z71, 4x4, with the Vortec 5300 V8 motor.
I've included a link to a video I took the other night of the problem. I easily accelerate from a standstill with no problem, but around the 0:20 mark, I hit the accelerator and the rattle becomes more noticeable, and the SES light begins flashing.