Steering Wheel Vibrates Only Idle In D

70 Challenger: steering wheel has a mild vibration only at idle in D at a stop. Driving, all is smooth no matter the speed.
If I have my hand on the wheel at a stop, I can feel the vibration. If I let go of the wheel at idle in D, the wheel vibrates.
What should I look for?
No knocks, no noises.
Engine mounts, loose steering unit? Do I need a rubber insulator on the steering column?

Thanks!

Way to little information. What engine, and trans, have you done a leak down, or compression test? Has it been tuned up in the past 20 years?

headers/exhaust hitting the steering box/linkage?

They don't hit the steering linkage but the passenger side header is close to the torsion bar. I just put the car up on jack stands today for the winter which gives me some more room under the car to check the clearances. I will double check the steering linkage clearance.
Thanks

a helper in the car with it idling in drive & brake on. You under the car & see what you see (vibration/touching)

I have a fifteen year old son that would fit that description. 👍 He is the only other person in my neighborhood who can start this car😀 Outside of me.
Thanks Robert