Car vibration in 1st gear and no throttle (when moving in traffic jams)

I have a VAG 1.6TDI 5spd MT octavia. I’ve often noticed that there is a lot of vibration when moving in 1st gear whenno throttle is added (as in traffic jams or whenever traffic moves really slow). After I add even a bit of throttle, the vibration is gone.
Could this be slipping clutch? I was under the impression that in this case the clutch slipping would be most noticeable in higher gears with no throttle, not in 1st?