Low flying disc shaped UFO

When I was approx. 12 years old we lived in an 2nd floor apartment that was above a store in Portsmouth, NH on Route 1. One night it was dark out. I was taking a shower. In the shower there was a small window that was open with a curtain covering it. As I was taking my shower I heard this strange low humming sound coming from outside the window. I was curious as to what it was. I pulled back the curtain to look out the window and floating next to the house above the tree line was a UFO. It was metallic, grey and disc shaped with moving lights around it's edge. The UFO was very smooth with no signs of seams, doors or windows. I do not know the size of the UFO because I could not see the whole thing. As soon as I saw the UFO it went straight up like nothing I have seen before. As soon as it was gone the low humming noise that was present the whole time was gone. I ran out of the shower and into the living room where my mother was and started screaming I saw a UFO. We have since moved from there. I don't think that I hallucinated the incident. My mothere remembers me coming out a telling her I saw it. The one thing that sticks in my mind is the noise that the UFO made that I will never forget.
Date is approximate.  PD