I often don't get into these noise problems as the question being asked often involves to many assumptions by the one asking. When done at the service desk a series of follow up questions is always necessary.
In reading the replies, I like the reply by Rex. The downshift from second to first is normally so soft that it is indistinct. If that shift is made with no vacuum the shift is a distinct "klunk".
I would verify the vacuum on the modulator. It should be around 20in on the 80-81 vac shifted 722.1 diesel trannies with the throttle closed. In this position the trans is in second gear. With the slightest throttle linkage movement the microswitch on the valve cover closes and the vacuum is dropped to around 12in at which time the trans downshifts to first.
Owner 24 bay BSC
Bosch Master, ASE master L1
26 years MB technician