I am not a professional but have many years experience fixing various mechanical things. I wanted to pass along a problem I fixed on my air handler that I thought might be useful or at least informational to other people.
First, I am not encouraging anyone to start poking around inside their air conditioner or furnace if you don't have some level of experience and don't know what you're doing. Big, moving parts can be very dangerous if they start up unexpectedly while your hands are in the midst of things.
My house is about 9 years old and the air handler is in the attic. A few weeks ago, I started hearing a throbbing sound along with the sound of the blower. I thought it might be the blower bearings or a balance issue. With the system turned off, I checked the blower and found the bearings to smooth and quiet. Also, standing beside the furnace with the blower running, there was slight vibration, but it didn't seem like enough to be causing the problem. To make a long story short(er), a paper label from somewhere inside the unit had come loose and was stuck to the inside of the squirrel cage blower on bottom side. It was hard to find because the motor was in the way and there wasn't much clearance between the bottom of the blower and furnace enclosure. I removed the paper and the noise went away.
Hope this helps someone.