2004 V6 problems (no codes show up)

Okay guys, so Ive been bouncing around the manual and online to find out whats going on with my car and so far I have many answers.

Heres the deal. My car starts a little funnier than it used to about 6 months ago. it seems like it takes .5 seconds longer to start. may not seem like much but its definitely noticeable. Once started it begins to idle, a bit rough and feels like its miss firing since it will thud randomly every few seconds. When initially putting it into drive/reverse it thuds when you touch the accelerator. when coasting it seems to not really vibrate but knock a bit. Once I push on the accelerator more it seems like it lags and sputters while also shaking (especially when uphill from a stop light, also when going uphill a strong odor is smelt, not fuel but it ranges from exhaust smell to rotten egg smell). When I hit the highway it sputters whenever I push the accelerator but if I keep pushing further it smooths out. A lot of the vibration and knock comes in around 1,750 rpm. Also there is a loud rattling noise when I approach a stoplight. Im thinking its my catalytic converter because when I crawled under I made a fist and lightly pounded on the exhaust pipes and it started rattling(of course the engine was off). That should be it, ohh and also there are no codes showing up on two different OBD 2 readers. also definite noticeable loss in power, feels like a four cylinder now.

So far what I've replaced are the spark plugs and wires( I've checked the plugs since changing them and they're properly gapped and theres no sign of anything like oil/carbon build up or anything else). I also replaced the fuel filter and have checked it since replacement and it seems fine. I checked my differential like someone said to and the fluid was fine. driveshaft is completely stable so I'm guessing its not the universal joints. I bought a cold air intake and its clean so its not that either.

So far the symptoms gave me a list of things that could be wrong. Someone said the EGR valve or system. Another said the catalytic converter. Theres also piston damage, faulty distributor, fuel system problem and others.

Sorry this seems so long and detailed but I love my butt stallion and I want to keep her alive. Any suggestions will help will help. Thank you