Sorry it took me so long to get back on here. Thank you both very much.

The problem occurs even when the locks aren't used.

I did try setting the temperature at 72 and the control to EC. The noise did go away. But then I noticed that it goes away when set at any temperatue and whether the EC button is depressed or not. As long as the fan is blowing, it doesn't happen at all! I'm puzzled but not as concerned since this will serve as a temporary fix.

Any other info would be appreciated. Does it sound like an air leak?