The 5.3 start-up rattle is normal. I had a 2001 Tahoe that was just starting to do it after 20k miles.

The 350 was past its sell-by date on meeting emissions. In theory the 5.3 should have its equal in terms of gas mileage, but Chevy delayed replacing the 4-speed auto then started giving all their trucks inter-galactic gearing to look better in the bogus CAFE tests. As the torque of the 5.3 came in a tad later than the old 350, this results in more downshifts in real driving and at best similar gas mileage to the 350 or often worse. WHat I did like about the 5.3 was its free revving nature. The 350 developed asthma above 2500 rpm, whereas the 5.3 would happily rev to the redline like a Corvette.

My Tahoe was doubly screwed in that it was CA emissions equipped. I have a sneaking suspicion that low rev "fuel-up" for acceleration is almost banished by the CA regs. Never again will I buy a CA emission vehicle, its 45 (?) or 50 state or the dealer can keep it. Unfortunately, the NV dealers in Las Vegas seem obliged to take CA spec vehicles from GM because of the supply chain. The same is true for Mazda of all people, too. I had a salesman try and tell me I had a have a CA spec car in Las Vegas. I hung up on him, eventually. His loss, I bought a Volvo.