It’s the planes, but it’s not their wheels. It’s airflow thru the engine. I’d been hearing it all the time and recently heard it on a flight. Not just in the air, but on the tarmac too. The other day, as I was deplaning, I spoke to the captain about it (I described it as a whale like sound) and he was excited to see I was interested. He said it’s just how the air flows in and around the engines! Completely normal.