imo it is a pack howl that you are experiencing. From my experience it is rarely heard by the pack leader, which must be you. This again is what I know from my experience and is not to be taken as professional advice. I had the same situation and the only way was not to leave in the end. Especially as they got older. If I was not in sight/scent of them the howl would happen but only at night. You might want to try CBD oil which might calm them. To my knowledge this is a stressful state for them pups to be in so that is the bad news. But the great news is that they obviously love you to pieces...