Vibration felt through steering wheel only at 5 degree angle

[2MCHPWR]

hi guys -
So at highway speeds, over 60 typically, there is no vibration through the steering wheel except when the wheel is slightly cocked / turned to the left at a 5 degree angle. Back at zero degree (straight ahead) the vibration goes away; same once you hit like 10 degrees.

The vibration is like a rhythmical thumping.

Why would this vibration only occur at a slight angle off center?