What’s the quietest thing you’ve ever noticed in a data center?

It’s not the hum, not the fans — it’s the silence between beeps, when a server’s been off for weeks but still draws power. I found an old cable once, no label, just dangling from a rack like a ghost. It wasn’t connected to anything. Just… there. I kept it. Not for use. For memory. What’s the smallest, quietest thing that stuck with you in a system? Something that shouldn’t have mattered, but did.