Crackling sound/noise from CyberPower PR1500LCD UPS

[sequoia]

CyberPower PR1500LCD UPS has recently developed annoying "crackling" noise. (Attached MP3 has sample of the sound)

This doesn't sound to me as typical inductor/coil whine (?)   Any ideas what this could be?

Unit is powering punch of equipment currently and passes self-test fine, so I haven't been able to take a look inside, yet.

Need to also check if this thing is still under warranty...although shipping might not be worth it as it weights over 50lb and I doubt warranty in US covers shipping (in EU/Australia it might be different story....)