This is on a hot water boiler? Do you have a pressure gauge on the system? What does it read? Depends on the system, but a reading around 10-15 pounds is common. The tank may be an expansion tank. If it is, my guess is that it may need to be replaced. Does it have a cap like on a tire on the bottom? If so, rap on the tank. Does it feel heavy and sound like it is full of water? Another symptom of a blown bladder in an expansion tank. Open the air valve momentarily on the bottom and see if water comes out. If it does, you've found the problem. They aren't much money, and are easy to install.
A picture needs to be converted so it is fairly low resolution, and the size is only as big as it needs to be to show the affected area. When you post a message, scroll down below the initial box to the line thatsays manage attachments...click on your pic file, and there you are.