Central a/c began making a weird noise outside, like a loud roaring sound coming from the condenser outside. What could this be? One of the rooms light flickers when this happens. Please help. The a/c seems to be running fine, except for the loud noise. Unit is 15 yrs old.

Does it sound like a race car outside? Most likely it is the compressor has been damaged and is now wearing down to the point of needing replaced. The only thing to quiet it down a little is a sound blanket. That won't fix the problem, just hush it a little.