fruitloop

So I installed berk high flow cats and a Megan racing y pipe. I have no check engine lights and the car seems to run fine. I've noticed that at around 1600 to 1900 rpms there is this vibration and rumbling sound. It's very subtle. I don't seem to notice it in 6th or 7th gear but seems it's at it's worst in first to 3rd gear. More noticeable decelerating but like I said it's subtle. I'm pretty certain the car never did this before installing the cats and y pipe. I don't know what I could of jacked up with the install, it was straight forward besides through everything possible to get the blasted demon bolts out. I was thinking a possible rich or lean mix at those rpms because the car is running fantastic. I don't thinks it's a drone, if it is it's the deepest drone I've ever heard...