Mine was indeed caused by the LP detector (Safe-T-Alert model 35-742).
Thanks to all who suggested I not just focus on the convertor. Was fooled by the proximity... would have sworn the sound was coming from there!
Thanks to the same folks who guided me that it might share a fuse with the kitchen appliances. That was the case. Pulled the fuse, noise immediately stopped, unscrewed from wall, snipped and capped the wires, then shipped it off to dealer for a repalcement. All is good!
FYI, in the meantime we took comfort in the plug-in model (9 volt backup) that my wife insists we always run as a backup! Heck, for $40 bucks, why not! Turns out her fears came true (i.e. faulty built in detector).
2011 Rockwood Signature Series Ultralite 8317RKSS