I have owned a 1995 S320 for 2 years and it now has 54,000 miles on it. About 3 months ago, when I started the car with the AC on (it's Phoenix, so it's always on), a whistleing noise is heard in the right passenger vent. The noise lasts about 5 seconds and then disappears. The dust filter and AC filter have been changed, but the noise is still there. It occurrs if the auto is cold or warm. The repair shop said they did not know the cause of the problem but have tried unsuccessfully with another Benz with the same problem to correct it without success and lots of $$ for labor. They said to forget it. Do you agree? Any thoughts as to cause or possible diagnostics?