A noise at power up is normal for pretty much everything with a self-selecting multi-voltage power supply, and that socket looks, well, pristine.
Symptoms of electrical problems include physical damage to cabling or connectors, degradation of materials, scorching, smoke, tripped circuit breakers, self-illuminating cables and connections, flames, and similar.
Without a similar Mac handy to check (and that can also be unplugged for a look), that looks very much like a C5 socket. The one photo here of the male connector only partially shows the labeling on what looks like a C6 male too, and what looks like a C5 female is unlabeled.
There’s a picture of a C5 female and C6 male bottom center here: https://upload.wikimedia.org/wikipedia/commons/0/00/Cable_Connector_Reference_Chart.pdf
But I see no reason for unplugging this iMac, nor for replacing any cables.
If you want an official answer, contact Apple Support and ask them to confirm the iMac emits a pinging noise on initial power connection. Or stop by an Apple Store, and explain your concerns to them, and ask if they’re willing to un-plug and re-plug a display iMac. (Preferably if it’s quieter than the stores I’ve been in recently, too.)
TL;DR: looks fine, seems fine, sounds fine.