when at around FL150 in torrential rain the rad alt was giving wildly fluctuating readings for several minutes.
A common problem with RA's is the electrical bonding of the antennae. If the sealant around the antenna is not so good, water can get between the antennae mating surfaces and cause indication fluctuations. Your aircraft may not have been reacting to the rain
below the aircraft.
RA's react to lakes, rivers, etc... Just how heavy was that rain?
Rgds.
NSEU