I have a '99 Chevy Lumina 3100 V6 with 54k miles

If the car has to be moving it is most commonly a noisy wheel bearing. That can happen on any brand of car but with GM's bolt-on type of bearing it can be impossible to tell which one it is that's making the noise. The only sure way that works for me is to run it in gear on a hoist, then listen next to each one with a stethoscope. The noisy one will be obvious.

Everything I can think of I can discount based on some of your observations. I was considering a rubbing timing belt or noisy idler pulley but your engine uses a chain. Perhaps the chain is stretched and rubbing on the cover. I've had a few axle seals that caused a high-pitched squeal and was wondering if a front crankshaft seal could do that too. That would occur any time the engine is running. You wouldn't have to be moving.

There is a tool you might be able to borrow or rent from an auto parts store that borrows them called the "Chassis Ear". It is a set of six microphones, a switch box, and headphones. You clip the microphones to suspect points, then drive around while listening with the headphones. You can move the microphones around to zero in on the source of the noise. Be aware that many mechanics have never seen or even heard of this tool. Suspension and alignment mechanics use it to find rattles, squeaks, and other noises.

The guys on the tool trucks sell them for $200.00. There's a newer model in which four of the microphones are wireless. I've seen the older version on amazon. Com for less than $70.00, but since you may never use it again, try to find one to borrow at an auto parts store.