Yeah, Dawn, I wonder too.   My daughter and I heard knocks in the middle of the night on the door first, then on what seems to have been under our windows a while back.  I did not put any importance to this until I started reading how more and more people are also hearing knocks...
Revelation 3:20

Behold, I stand at the door, and KNOCK: if any man hear my voice, and open the door, I will come in to him, and will sup with him, and he with me.

This verse is the one that seems to relate to the knocks we hear.  Perhaps the Lord is giving us comforting words that He will be coming soon and to be prepared.  I certainly went to see who was knocking and opened the door but like you all there was no one that I or my daughter could see.  Interestingly, we have 6 to 7 people (some work the late shift) at home at night and only the two of us heard it.  Not sure what it means, but I will keep my ears as well as my heart open with a plate ready for My Lord to come in and dine with us.

We are ready Lord, to do your work and follow you.

Barbara