It wasn't a fan.  She has her monitor along with
various picture frames and such on the top of her
optiplex, which was causing the top panel to
vibrate.  I took off the panel and reattached it,
and adjusted the placement of her monitor and
pictures. This stopped the panel from vibrating. I
let her know that the items she was placing on her
machine was causing the vibrating so she doesn't
have to call us again in the future. 

She said that she can shake or tap the computer 
and the noise will stop for awhile and then it 
will start back up again. She is in all day at 
the front desk of the Health Center. She said 
that the noise has been like this on and off for 
about 3 weeks now.