The title says it all. I can't find it in any of the standard references.
PMEG has a list of unofficial interjections which might be helpful. Perhaps this one would be appropriate:
ba, baf = malŝata senzorgeco
However, I think it’s unlikely that this would be widely known and in practice the meaning of the word is probably mostly expressed just in the intonation and the facial expression. With that in mind you could probably get away with just saying “ho hum” and even people who don’t speak English would understand.
If you are in a very international context and want to be very clear perhaps you could just say an actual word to express what you mean such as elrevige or tede.