I have a 2004 automatic 3.9l mustang and it makes a rattling/chattering sound that sound like its metal on metal from the engine bay and it sounds like it is directly in the center of the engine. This sound only is present when the engine is in 3rd gear or higher and only happens between 1500-3000 rpms. It also will only happen if I hold my foot in a certain throttle position. If I let off it will stop and if I floor it, it will stop making the sound. There is no power loss at all or no noticeable changes besides sound. Also, it does not make the sound at all at idle or in first or second gear. The only code the car has is a p1336. I replaced the camshaft sycroniser assembly and the camshaft sensor. However, it did not make the sound go away. Please give me any ideas of what it could be.
The first thing you want to eliminate from the list is a catalytic converter shield rattle. Do that and check again.
Sounds like the timing chain may be worn out. You don’t say how many miles are on the engine but a high miles Essex V6 can wear out the chain and cause noises. Listen from the engine compartment and have a buddy rev the engine in neutral so you can listen.
Odd that it is 3rd and 4th gear only. Loose heat shields certainly could be the cause.