You are likely getting interference from something nearby:
In the presence of electrical interference, you may experience frozen images or intermittent audio while viewing over-the-air television programs. This interference may be caused by equipment in your home, such as hair dryers, sewing machines, electric drills, doorbell transformers, light switches, smartphone chargers, power supplies, computing devices, washing machines, clothes dryers, fluorescent lights, LED lights, or garage door openers.
Electrical interference may also be caused by power lines. Interference caused by your power company's electrical equipment is normally continuous, and your power company should be notified.
A simple method of determining the location of electrical interference is by using a portable battery-powered AM radio tuned to a quiet frequency at the lower end of the dial. You should hear static or a buzzing sound as you get close to the source of the interference. The closer you get, the more intense the static will be.
If you're in an apartment/condo, it could be from a neighbor. If your antenna is near a wall, my guess is that there are electrical wires behind the wall or outside the wall.