That vacuum light indicates that the compressor is running for a long period of time. Usually the fault is that the condenser is clogged with dust. You can try vacuuming it, but it doesn't necessarily mean the condenser is clogged, that light is like a check engine light on a car, it just means something is wrong with the cooling system.
If you have a knocking noise and it stops when you open the freezer door, this leads me to believe the issue is the evaporator fan. This fan turns off when you open the door and is responsible for circulating cold air throughout the freezer. If it is noisy, it is either defective or hitting something that is causing it not to spin efficiently.