You're in the gym and the person next to you makes lots of noise doing the exact same exercise that you are doing silently. He is not sweating more than you are, he is just loud.
You're walking along the street or sitting outside having a cup of coffee and a car goes by that is deliberately so loud that you cannot hear anything but the car. (This is not to be confused with a car I had whose muffler seemed to rust through every spring. The car in question is loud on purpose.)
You are in a conference room waiting for a meeting to start and someone is complaining very loudly how nothing ever works in IT and that he/she could do a better job of "doing IT" than the IT team does. This person is going on and on and has not just made an off-hand comment that Windows seems to blow up more than clown's balloons at a child's birthday party. To coin a phrase.
I call the sounds occurring in each of these three cases, "LOOK AT ME" noises. In the first, the person wants you to appreciate how hard he (in my example) is working. There is nothing to do except ignore that person and keep a wary eye that he does not hurt himself and need you to call 911.
In the second, the person more likely than not is compensating for not getting enough attention in other ways. There is nothing for you to do in this case other than to roll your eyes and plug your ears.
The third case is harder to characterize. When you hear this kind of talk, the first question you have to ask yourself is whether it could be true.
Is it true that critical IT functions are not available as much as they should be? If so, you should acknowledge that that is the case. You should enlist the person's help after you believe you have things under control to update his/her opinion of IT and to be a champion for you. Then you should get back to work.
If you are meeting your SLAs, and this person's criticism is not true on its face, then you need to keep digging. It is unlikely that "nothing" works in IT, so look beyond the hyperbole and ask whether the most important things for that person do not work. This is a different situation than "nothing" working; it is a question of priorities. Set out to understand what you are missing. You should do this privately so as not to put the person on the defensive in front of others.
If you think you understand the person and his or her requirements, ask yourself whether you have deployed the right capabilities for that person. You may think you know what the person needs, but do you? Follow up with the person afterward to see if the point is that the system does everything except BLANK, with BLANK being most important to that person.
If you have confirmed you have deployed the right capabilities and you know you are operating them effectively, ask yourself whether you want to follow up after the meeting or whether you want to call the person out now. My point of view is that it is right 99% of the time to follow up privately afterward, but at some point, the only way to correct a person making LOOK AT ME noises publicly is to call that person out publicly.
Before you call the person out, think about whether you can win the fight and whether the fight is worth winning. Crushing a junior person like a bug is never appropriate. It also is rare that embarrassing your boss is a good idea.
Assuming you decided to follow up after the meeting, first sleep on it. The follow up conversation may start out contentious and you want to take any emotion out of it. Then schedule a meeting. 45 minutes is enough since if it goes badly, you have only spent 45 minutes, but if it is going well, you have a good chance of getting 15 more minutes. Pull together the last quarter's IT operating results. Refresh your memory on the data flow of the system that the person was complaining about. Go to the meeting.
Set the tone that you want to understand where you and your IT team are coming up short. Keep calm. Listen more than you talk. Take good notes. Commit to action items with dates. After the meeting, send a thank you email with the commitments you made. Follow up on them.
There are some people who are just happy being unhappy and loud about it. If it turns out this person is one of those people, then reconsider whether you address the issue head-on, in public, the next time it comes up. Most people are not happy being unhappy, however, so you may have been able to get this person to stop saying LOOK AT ME and say LOOK AT OUR RESULTS.
As always, good luck!