Go running over and Exclain "Gosh, There must be something wrong with your unit, ahve you heard this nopise? Maybe you need to come around here and check?"
- and then excitedly lead him to the unit...
To take your neighbors side, it could be that he is unaware of the noise and would appreciate being told; it could also be that his utility bill has DOUBLED, he doesn't know why, and would APPRECIATE being told! (You might even suggest he check his bills?)