'04 grand Cherokee a/c problem: sounds like a horn blowing under the dash
 

My a/c is now working intermittently. Standard a/c, NOT the automatic zone controlled business. Sometimes it's cold (not super cold, but maybe cold enough), then sometimes it will just quits blowing cold. But it is occasionally making the strangest noise under the dash. It will suddenly sound like someone is blowing an air horn. If i hit the a/c button and then off the compressor then it stops.

What the heck is going on? I know just enough about a/c to be dangerous, but I've got no ideas here. Any suggestions?

When it stops blowing cold, check under the hood to see if there is any frost on the large line from the firewall to the compressor. If there is, it is undercharged and freezing the coil with solid ice