My first indication was the exhaust manifold cover. I know on some Toyota engines in the past the metal to metal sound was the metal itself expanding and contracting as the engine warmed up and cold down. Since you said you just had work done, I would suspect that maybe the cover or exhaust system is loose. You mentioned the noise coming from the rear, maybe when they were working on the exaust manifold they knocked loose or broke a bracket off the exhaust system where it mounts to the frame causing the metal to metal sound your hear when letting of the gas. Also, possible the tech forgot to install a bolt or two in the brackets themself. I would have the dealer who did the work check it out.