funky smell/taste

[showrguy]

hi all, when i dilute my rum down to 40% or so it gets this strange smell ??
kinda hard to explain, sorta musty i guess you could say.
at 75% it tastes rather sharp and has a sweet aftertaste, when i mix with coke it changes ???
i was using a cork automotive type gasket between the still column and the boiler, it probably has some type of glue/rubber in it to bond it together.......could that be the culprit ?? (it never touched the spirit)
i wiped the still down and flushed it out after the previous run, dunno if it might have been dirty ?? i'm puzzeled...
thoughts........