PS

*ḏVnVm-/ḏVnVn- - rain (DRS 336; Kogan 2011: 193–194)

Probably *ḏnn is the original shape of the root dissimilated in EthS and Sab, but the reverse possibility (an original *ḏnm assimilated to znn in Akkadian) cannot be excluded either (cf. DRS 336)
(Kogan 2015: 442-443)

Akkadian
zanānu - to rain (CAD Z 41, AHw. 1509)
Cf. zunnu 'rain' (AHw. 1537).
Hebrew
zäräm - rain (HALOT 281)
may be related with dissimilation
Sabaic
ḏnm - rain (SD 39)