Hey Sweetie,
Just read your original post. With that said, I understand what it is like to live in an apartment / apartment complex. Strange and new sounds from time to time.
With that said, try some physical tests first. Keep in mind that apartment structures, regardless of the rent being paid are slapped up relatively quickly so as to turn a profit.
Check the seals around your doors and windows. Even the slighest of drafts or breezes can cause tapping sounds to these areas.
Do you have bushes, shrubs or trees near your building or doorway? Again, the slightest of breezes can cause sounds.
Look at the outside of your apartment. Do you have anything sitting outside that can be moved with wind, etc. This includes grill covers, lawn chairs, wall / door hangings, even a door knocker if it is loose.
Because you live in an apartment complex you cannot do background histories of deaths that occurred on premises. Records on file are not the same as if you lived in a single family home where you could research the plot #, etc.
Also, look around for signs of rodent activity on the outside of your building. Even little animals can find ways to make knocking and tapping sounds.
If all of the above have been ruled out, then consider the possibility of paranormal activity. Things you can do:
Go to a local library to their geneaolgy section. They will have maps, and other documents of the area before homes / apartments / stores etc were built. Who possibly use to live in the area or on the property where you live before it was built to what it is now. This is not a find out the answer in 10 minutes. This will take some time and carefully going through old newspapers, documents, maps, microfiche, etc.
Document thedates and times and locations you are hearing these sounds. Is it repetitive at a certain time or is it spontaneous. Take note of the activity going on outside your apartment when it occus. What is the weather doing? All these things factor in.
Do you have a camcorder or other filming / sound recording device. Film the areas being affected. Ask questions. If you are not big on technology with a computer, you can always download Audacity for free and run the sound recordings through there for possible EVP pick ups.
Are you feeling it is a negative energy doing this? Are you being harmed or is it just a nuisance?
Just some thoughts.....