Is it wrong of me to let the baby cry?

I'm a nanny, and I consider myself to be a very good one. I've been doing it for over 5 years for kids of all ages. I started a new job last month watching two twin infants. (4 months old) They're really easy to care for, and I haven't had many issues. But the thing is, they cry almost all the time. Even while I'm holding/rocking them.

Now, I've cared for many different children, and know they're not all the same and you need to use different techniques. I've tried holding them and rocking them in different positions, using different sounds/sights, etc... I literally just changed them, fed them, and put them to sleep for an hour. They're not due to eat for another 1.5 hours. Burped and gave them their gas drops as well. They're both screaming, and I know there's no real reason for it.

They don't do this all the time, and they'll get distracted and stop for about 15 minutes, then start again. Usually when this happens, I'll just let them cry it out. Their parents say the same thing. No real reason for the crying, but yet they'll scream their heads off and it can't be helped. I love them like my own, so of course I cuddle them and hold them. But there's also two of them, and I'm just one person. If I'm doing something with one, the other will scream. I'm a nanny because I love children and I love what I do. These boys are a little bit of a difficult case for me, and I'm on here looking for advice because I really am trying.

But I still feel awful for just letting them cry. Is there anything else I can do? It doesn't seem like colic- I've seen it before and it's nothing like this- but it's still an obnoxious amount of screaming.

Is it wrong of me to let them do it? I'll usually just sit near them while they get it out, because I don't want them to rely on me or their parents picking them up and holding them EVERY time they make a fuss.

Edit: Their parents have told me it's best to let them be. I've given them what they need, and I know they're not hungry, dirty, or sleepy. When I let them cry, I'm right next to them talking to them so they see and hear me. It just feels awful that there's nothing I can really do otherwise.