Cat making strange noise

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 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.