One cat would yell for a while, then settle down. When she was older, it took longer for her to settle down, until she was just yelling constantly for hours. I finally gave up and let her out of the carrier - the only time I’ve ever done that, and we were an hour from home with no traffic and no stops needed - and she stopped yelling immediately, just stood on my lap, panting.
At which point I realized that, as large as our car carrier was (it’s one of the largest ‘carryable’ ones I’ve seen), it wasn’t quite tall enough for the cat to stand completely upright and she was in pain because of her arthritis. We’d been torturing her, for hours! We made some changes after that!
For people who have carriers with a smooth or only slightly rough bottom: your cat may be yelling because they can’t keep traction and are unexpectedly sliding a bit on curves and turns. We got one of those non-slip shower mats and cut it down to size, to reduce slipping, then added a beach towel for comfort. Everything is still washable (use white vinegar to get rid of the smell of cat urine), but the cat is much happier!
Ours is an old carrier. The interior dimensions are 16 inches tall, 14 inches wide, and 24 inches long. Our cat was maybe 14 inches tall, but of course she wouldn’t want to stand fully upright and either bang her head on the roof, or constantly brushing her ears against it. Plus the bath mat and towel ‘raised’ the floor a bit. And sometimes when you scream, you just want to throw your head back and let go!
I like the bath mat because I can suction it to the bottom of the carrier, so neither she nor the towel slide around like they would on plain plastic or metal.