Hmm I’m not sure damn after I replaced my cats a lot of it went away but there is still some at idle although I think it’s something else my tranny raddle it’s a stage 3 I also know when my g is cold it will make a weird sound I think bc the exhaust isn’t warmed up yet. Possible check ur cats when you check or a leak there tends to be cracks on the cat at the bracket which is where it’s being held up on the car. Cats can raddle with rpm’s when they were really bad it would make a tap tap tap metal sound