98-22-4 this is the number of the TSB on the noise that these units make. It can be found at
http://www.ford-trucks.com/tsb/tsb.php. You will have to join the site to see the full text, but it is free. I joined because I thought that it would be of help in the future. I just bought a 1999 Pace Arrow on a Ford chassis, and it also makes the noise, which sound like an air or vacuum leak.