Yes you could possibly still have a bad bearing even if you find no excessive play or noise when checking it. The sign is the increasing noise when you drive it. As you drive it could be heating up melting lube out, you get friction and expansion of parts, tolerances become less noise starts and increases.
I would first eliminate all other possibilities.
Make sure the noise is coming from where you think it is.!!!!!!!
If you have eliminated the pebble possibility look for other likely causes.
Brake pad wear indicators? bent backing plate? something rubbing on the axle? loose mounting bolts at axle to transmission.? If you eliminate every other possibility
what are you left with.????

You have to listen carefully as bearing noises can be hard to ID, usually they rumble under load, like if you turning hard to the right, you should hear a sound change on the left wheel..

Hopefully what I am trying to explain makes any sense..

Totally makes sense. If I can’t find anything I will go over to my local Porsche mechanic and see what they can find.

I do have one question, how dangerous is it to drive if the sound is so intermittent and it goes from no sound to the horrible squealing sound? I’d like it be be doing it when I take it to the shop.
I got to think if the bearing is compromised it should get hot and eventually come apart.

"Horrible squealing" is a clue. But your Horrible squealing could be my "Hmmm that does sound right" wheel bearings don't usually come apart they seize up.

When I have an issue with an automobile that I think is serious. (more so with engine or trans issues). I think long and hard before I drive or even start the car. (If engine related). Why???? Because all the moving parts are inter related one simple issue can become a blown engine or trans.

If what your hearing, seeing ,feeling, smelling, thinking, is wheel bearing, then you have to make a judgment call. drive to the shop or not.

how far is it? hiway or city streets? you can try it. If it starts to squeal pull over feel the hub for heat. let it cool down if hot. try again. Or you may just make it no problem;)
What ever happens let us know what you find. that helps us all learn.:D

If the tell tail signs of a bad wheel bearing are not there (play in the wheel, noise when you spin it on a jack) would the wheel at least be hot after a drive?

If it is the wheel bearing, and there's no play in the wheel, it sounds like it's in the VERY early stage of failing. I've driven daily with bad wheel bearings for months after I first noticed any noise, and it was making constant noise. Like Blue62 said, they generally don't come apart. They CAN come apart, but if they do, they have to get pretty bad before they go.. If I were you I'd first check for a pebble. If there's no pebble I'd keep driving it until the noise becomes more predictable before taking it to a mechanic.

This is a funny vid. I've never seen wheel bearing so bad that a driver in a DIFFERENT car can hear it!

https://www.youtube.com/watch?v=_lzd0gTwreQ

If it was squealing during the drive and you check the hub for heat I would think yes it would be hot or at least warm depending how far you drove it while acting up.

My question is..... are you 100% certain the noise is coming from the rear wheel?????
After all I assume you are inside the car when you hear it.

Could it be something else????? Water pump? power steering pump? Idler pully? Alternator? All those things are right behind your seats. sound travels.

take it for a drive and get it to squeal. see if you can tell if the sound changes with different engine RPM. see if the sound changes with different road speed.
If you have a standard transmission see if the sound changes with clutch pedal in versus out while moving.

The last time I drove it and it was louder than usual I thought it was definitely the wheel making the noise. It certainly modulated with rotation speed of the wheel but didn’t change when the brakes were apply or when I went around corners. It was so loud I thought for sure it would still be doing it the next day. But nothing when I drove it. I will be home tomorrow and will drive it and hopefully get it to do it again. I’d like to get this resolved before I take any kind of trip.

Keep us updated. Always interested in outcomes.

Will do. I appreciate all input

Something nobody has mentioned is the emergency drum brakes. If one of the shoes gets worn down to the rivets, or the return spring breaks, or ..., you can get the most horrible noises from them.

That is a good point

+1 excellent point. WRS, if the noise comes back, you may want to fiddle with the e-brake handle while coasting to see if the noise changes. I didn't even think about the shoes, I didn't realize they had drum e-brakes. (Porsche newbie) Actually, even if it's not making noise now, you can pull up on the e-brake handle momentarily WHILE HOLDING THE RELEASE BUTTON when coasting, and if the shoes are worn to the rivets you'll hear them.

An easy way to test your theory, is to drive it and slowly pulling the parking brake level to see if the noise returns..

there are two interesting things about this noise problem from the rear wheel.
(if it is the rear wheel)???
1. It was checked out by Porsche mechanics and no problem found. ……….
2. Comes and goes.
The E-brake idea is something I wasn't thinking of, good call.
I would think a half decent mechanic would have checked it though.
It is something the OP can check in the way described and perhaps find the problem or check it off the diagnosis list.
keep us posted;)

Over time, I replaced all 4 bearings on my 2000 Box S during ownership. A couple presented themselves in typical fashion and were easily identified. Two others were very subtle....no unusual noises while turning one way versus the other and no play felt by pulling on the tire. Anyway, all issues were solved by new wheel bearings, with all work happening after 100K miles.''

This car only has 39k. The emergency brake theory makes the most sense. The noise was not effected by my braking. I too think a good mechanic would have checked this. I thinking the previous owner knew about it and didn’t want to spend the money to fix it. If the emergency brakes need to be fixed I’m assuming they would do both? If so is this as much as a wheel bearing?

With 39k on the car the E-brake should have very little wear on the shoes if any. and should not be a problem. E-brakes are not usually used to stop a moving car.
I only set my E-brake after I am parked. So mine should never wear.
Now if someone drove with it applied or partly applied or abused it in some way that's different.
But testing it is easy: slowly apply it while moving if there is a problem with it you will know it.

Now that I see you only have 39k on the car I wouldn't think e-brake or wheel bearings would be a problem.
But being a 1999 that means it sat for extended periods of time. lots of strange things happen to cars that sit for long periods of time. My 2000S with 47000 when I bought it last July had no rubber left in the motor mount, rotted out mostly from not being driven.