F-150 rattle

The check engine light came on my 2001 F150 v6 with automatic transmission 4x4 at 80,000 miles. I had 2 O2 sensors replaced because it was running lean. After repair I have an engine chatter or rattle similar to valves rattling but it is not. I put 89 and 91 octane in truck and it did not help. A mechanic changed oil to 15w40 and that did not work. The Ford dealership said its the catalytic converters but they do not rattle when you hit them with a rubber hammer. Midas agrees it is not the converter. The rattle is louder when pulling a trailer but can be heard without a load. Can you help me.