Entire car is vibrating when tachometer is between 1 and 1.5 on automatic trans

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My Elantra has 167k on it. I actually had a new transmission put in at 38K because of a factory defect. Havent had any problems since. When I am giving the car gas and the tachometer is between 1 and 1.5 the entire car vibrates. When the car idles it does so below 1. I didnt think anything of this until my friend was teaching me how to drive manual this weekend, and the same feeling I feel in my car is the same vibration i felt if I let up off the clutch too much before her manual would stall. Its not a violent shake - its that vibrating that happens right before the violent shake and the engine stalls out in the manual. This transmission has not been touched since it was put in and has 129k on it. I am wondering if I need to get a transmission flush but I was reading if there are already problems with the transmission, a flush might exacerbate those problems. Any help would be greatly appreciated.