Even though your team members are adults, they might still like to whine and complain. This is human nature, but it doesn’t have to derail your group. When people on your team get frustrated and need a sympathetic ear, do the following:
Examine your role. Why do people come to you to whine? What messages have you sent that make people feel that this is acceptable? Do you always solve their problems? Do you jump in and mediate so the team can be without conflict?
Insist on accountability. Don’t allow people to present a problem without attempting or providing a solution. It's easy to be the rescuer and mediator. Still, you aren't doing them any favors by getting involved unless necessary.
Encourage positivity. If the whining is about fellow teammates, encourage the group to work with each other to understand the nature of the problem better and then create a joint solution. Remind them that most people aren't intentionally difficult.