ok my car has been running preaty bad missing alot changed the plug wires and ran a little better changing the plugs this weekend its sort of chuging to loss of power now i hear a rattling sound like its coming from the cat converter would that cause a rattling sound it goes away at higher rpms i looked at it while the car was running and it was smoking alittle so im preatty sure its clogged just wanted to know if that what would be causing the rattling sound.

 

I believe some cats have catalyst agent that is actually in the form of a brick inside the converter that exhaust passes over and is converted into "acceptable" gasses. The catalyst inside the converter shell could have broken up into pieces.

 

I think most of the cats now have the brick style catalyst and I have heard of them breaking, with the chunks causing rattling.

 

ok the rattling goes away and comes back every now and then the car chugs like chug chug chug chug and when i hit 55 and stay at that speed for awhile the check engine light will come on but when i slow down it goes off could it be my o2 senser to.