4 Wheel Drive/SUVs: stange noise when heat or air conditioner is running, oxygen sensors, vacuum hose

Question
I've been trying to track down a strange noise coming from the engine of my 1996 ford explorer.
this happens when my air conditioner is running in summer time or the  heater in the winter..the noise even gets louder or weaker  when you turn the temp from different positions..
it sounds like a horn blowing but a very sick or faint one. we have taken it to a local ford mechanic and he told us that it sounds like the manifold seal is starting to go bad  and its sucking air. he said it runs too smooth to worry about it at the moment, but although the noise is not as loud as a horn when you are parked behind someone and it begin to sound off the people in front think its a horn. i have experienced 2 different times when this happens the engine tried to die...I am afraid to take it on a long trip fearing a break down so I would like to get it fixed....other have suggested a vacuum hose may be the problem.......it does not do this the whole time you are running the engine but after you have put a few miles on it with the air or heat running then it will begin......we have changed the airflow filter, which was costly and a two oxygen sensors after the check engine light came on and the code reading showed it but neither fixed this problem so it must have been unrelated to this issue.
if you have any ideals please let me know thanks Larry

Answer
I have already answered this question, but here is my answer from the preveious one "well you could try getting a a/c line test kit, or recharge kit that comes with a black light and leak detecting kit, and at night do it and run the a/c with the vehicel running, and use the blacklight to check each line for leaks."

hope that helps.