Sorry for not having been clear - it's nothing in the cider itself, but rather that the barrel has sat empty and dry all these years. Wine makers store empty barrels with sulfited water in them to keep the staves swollen and to discourage microbial growth. And they traditionally "sweeten" them before refilling by burning a sulfur candle in them, filling them with SO2 fumes.Why would previously barreling cider prohibit you from re-barreling whiskey?
So I suspect that my barrel may be musty. Obviously nothing would grow in it if the alcohol level is sufficiently high, but it might taint the contents.
I have been considering, however, checking it out and if it's clean, filling it with fresh make white dog from a 50% rye malt, 30% unmalted rye, and 20% distillers barley malt. I believe I might find connections for such a spirit. (I discovered in a test mash that Briess Malting Co.'s rye malt, unlike barley malt, lacks sufficient enzymes to convert even its own starch, so additional enzymes must be added from barley malt. Apparently, Anchor Distilling gets a higher enzyme rye malt.)