Engine or transmission mounts are my first guess, if sagging or improperly installed (tightened in non-settled position), next would be the possibility of the exhaust touching one of the hard-rubber bumpers under the car.

Idle it in drive, then idle it in reverse, see if there is any difference in "roughness" or engine sound transmitted into the chassis. If quieter in reverse, right engine mount, if in drive, left engine mount, either could indicate exhaust rubbing.

Just a hunch based on my 60x / 124 experience, might not transfer to your model.

__________________

Gone to the dark side

- Jeff