Stock 340s had relatively big cams (long duration with 44 to 52 degrees overlap) in them and they did idle rougher than most other MOPAR engines other than HEMIs. Stock ones ran great and the stock intake with the AVS worked very well and still do. At 750 to 900 rpm, they do thump a nit and with headers it should be noticeable. I doubt though yours has the stock cam if someone has already installed headers and changed the intake.