Blast, I remember making a bullroarer out of a ruler as a kid.
I never really thought of them as anything but a toy.
After searching on the internet and finding out more about them it seems like a possibility.
I am going to try making a few different ones to see how loud they can be and how simple they are to make.

It might even be possible to make one into a multiple use item or out of something that would be normal gear. (Hiker's trowel?)

Wish me luck.
The videos I saw make them seem very loud, but I never know if that is really true or not until I do it myself.

When I was a kid, they taught us in school to fire three shots with a rifle if we were lost. No kidding at all. Times have sure changed. I believe they still teach this to hunters, who might recognize it as a distress call. Maybe. So many people shoot randomly that it's possible nobody would pay any attention.

I don't know how easy it would be to locate, but a rifle shot carries a long way.

Don't forget the old car horn.

A young girl followed the trail back to a parking area in Oregon and blew the horn in threes with a pause in between each set. Someone heard the pattern and recognized it as a possible distress signal, and followed to the source.

Rescuers went back down the trail to where her mother lay down a slope with a broken leg.

You use what you've got and keep your fingers crossed.

Sue

Breaking a branch? The loud "crack" broadcasts a long way. I would think that any repeating noise. If I was listening for someone I would listen for un-natural sounds, like leaves crunching, but that does not carry far, maybe a quarter of a mile. You can make a whistle pretty easy though. A blade of grass or paper held between your thumbs, blow in the hollow (remember when you were a kid?) The cap of a sharpie or pretty much terminal tube. video of sharpie whistle Half cover with your thumb and hold at a 45 degree angle against your bottom lip. monkey squirt or helicopterfan held between your tongue and the roof of your mouth. (kid trick again)

Hitting things together can provide some kind of sound signal. Rock on rock, wood on wood, metal on metal. Hitting something hollow better than something solid, but "needs must."

What about hitting your cooking pot or kettle with a spoon, stick or whatever else is handy?

Hold or hang by the handle; don't grip it tightly at the rim because that will tend to deaden the sound. Try hitting it in different places on the bottom and sides to find the "sweet spot" (where an object is most resonant).

A metal water-bottle should work too. I'd guess that stainless steel would give off more sound than aluminum, but I haven't tested this.

Some pot lids make lots of noise. Again, hold or hang it by its handle and experiment to find the sweet spot.

A neighbour is building an addition a mile away. I can hear the sharp crack of every hammer blow. Woodpeckers use the same strategy to mark territory with sound. So the back of a hatchet against a solid, dead tree could certainly work.

I also like the idea of a steel spoon on a steel pot. Forget aluminum though.

Used to be that people in bear country carried a pen launcher, with a bunch of bear bangers and a few flares.

I have seen air horns that have their own built-in pump. Seems to me that sort of noise would carry, and get attention. Possibly the best alternative.

I've never used a Jetscream whistle. Wonder how it compares.

I should add that my hounds don't really respond to the Fox40 whistle. It takes a lower pitch. A flat whistle brings them in much better. Go figure.

P.S., Following up on an older S&R thread: "If a bra falls in the forest, does anybody hear?" (Bad, bad Doug. Goes and stands in corner.)

Yes, Doug, exactly the kind of awful stuff that could produce low frequency groans!

The noisemakers that I didn't see mentioned include gunshots, tradition is three in a short time but even one tells people for a considerable distance that someone is there. In this a shotgun, for obvious reasons, would be better than a .22LR.

A vehicle stuck in the woods metal parts might be banged together to make noise. Even better if you can suspend a piece so it rings like a bell. Perhaps from a length of wiring torn out or a seatbelt. Older vehicles, with their larger selection of stout metal parts, like hubcaps, might be better than late model vehicles with less steel.

If you need to get someones attention the single most effective sound may be breaking glass. Whistles and shouting 'fire' don't evidently carry much weight in an urban environment. The sound of breaking glass was judged to be more likely to cause someone to investigate and/or call police.

Use some kind of tube as an improvised wind instrument / trumpet.

Blow across bottle or jug partially filled with water.

Blow across part of a nitrile / latex glove as an improvised shrill whistle.

Modify a can filled with some water on a fire or a stove to create a steam whistle? [Not sure how to do this.]

"Crack" an improvised whip.

I carry a couple of whistling pete's left over from the 4th. of July.
If I set one off I bet someone will be along in short order to give me a ticket.
Sparks from these are not a big issue if you use reasonable caution.