My air handler is mounted in the basement directly below the 2 upstairs bedrooms. About 2 weeks ago we started hearing a shrill squeal from the floor vents in these 2 bedrooms. The squeal died down to a rattle. I have traced the sound to the air handler. The noise is very irritating in these 2 bedrooms because the duct work is very short from the handler to these 2 rooms. The sound is not very noticeable in other rooms where the air has to travel farther to reach the vents. The unit still seems to blow cold air just fine.

What is causing this rattle and is this an easy repair or best left to a Pro?