Cat making strange noise

I know the sound you mean I think, it certainly does sound awful.
My cat was 15 and he was ill when he started yowling. I always associated the two things - but if the vet's found nothing abnormal on exam or bloods then maybe it's not pain.

I think I'd keep a record of when he does it, and any other odd behaviour, & take him back to the vets if he's still doing it in about 3 weeks' time.

I am sorry your cat is doing this, it must be so distressing for you and him. I have had cats all my adult life and the one who started yowling had dementia.
On the other hand it is very hot so maybe he is just uncomfortable.
If it persists especially if he is losing weight or his coat loses condition I might go back to the vet. It’s good his tests came back clear so that would rule out diabetes, kidney issues or a thyroid problem but it would certainly warrant a second look.
I hope he settles soon.

I was going to say that my friends cat started yowling when she got dementia (the cat, not my friend!)
Anytime she was alone in the room, sitting in front of a full dish of fresh food, and at the cat flap, wanting to come in.
My friend had to go out into the garden and "post" the cat through the flap!

It’s sad but it’s a senile thing that cats do when old, my cat aged 15 at the time would go out in the yard and howl, the only thing that stopped him was picking him up for a cuddle.That made him realise where he was and who I was.☹️

My Miss Kitty started this when she was about 17. She would go upstairs and get lost. It was so distressing when she got dementia.

My almost 21 year old cat does this, I can hear him from way up the road! The vet says it is common with elderly pets. He is deaf and appears a little confused at times, but generally content. He also drinks a lot because his kidneys are not great, but no medication except for hyperthyroid. I have water bowls in various places for him, but he ignores the one in the kitchen near his food!
There was a time when he was only vocal when bringing in a live 'present'. Fortunately he hasn't done this for a few years.

My 15 year old cat yowls before she brings up a hairball.