Diagnosis
It's probably not broken, but it is a known audio hardware artifact. A faint, high-frequency hiss or static from the tweeter when idle is reported by a number of users and is generally considered a design flaw, not a failure.
How to Fix It
First, check if it's actually idle. Is it waiting for a 'Hey Siri' command? You can turn this off temporarily: go to the Home app, tap the HomePod, tap the gear icon, and turn off 'Listen for "Hey Siri"'. If the hiss stops, that's the cause—the mics are always listening. You can turn it back on; the hiss is the amplifier circuit powering that system.
If That Didn't Work
If the hiss is loud, audible from across the room, or changes with volume, it could be a defective unit. Try a different power outlet and adapter to rule out electrical noise. If it persists and is bothersome, contact Apple Support, especially if it's under warranty.
Heads Up
This hiss is often more noticeable in very quiet rooms. It's not something a software update can fix, as it's related to the physical amplifier components.