Rattle on slow acceleration

What you described is more often caused by a loose exhaust system heat shield or rusted clamp. Bang on those areas with a rubber hammer while listening for the noise.

That's very common. The exhaust system was most likely removed and other parts were shifted around. Shields often crumble from corrosion around the bolt holes. Even a little momentary pressure on one can make it stop rattling until vibration or more corrosion cause it to become loose again. Loose heat shields that were welded on each end to a pipe can be tightened with a hose clamp. Shields bolted to the underside of the floor can be tightened by removing the bolt or screw, then adding a large-diameter washer under its head.