Good news that your noise is gone.
If it returns after your batteries are used some, I feel the most likely cause is a failed fan as mentioned above. I would suggest however, that replacing a simple fan would be far cheaper than replacing the whole converter. Using this as an excuse to upgrade the "less than amazing" original converters is another story though!
'04 GMC Sierra 2500HD, CCSB, D/A, MaxBrake, B&W Companion, Retrax Pro, AR Power Steps
2013 Rockwood 8289WS w/Diamond pkg
1969 Camaro RS/SS Convertible
Dad's old 1968 Empi Imp (recently recovered from under 21 years worth of boxes/dust)