Strange noise when AC is running

I have a very strange problem that I hope someone can help me with. My 99 5.2L SLT Durango has started making a strange sound that I can only describe as a badly out of tune horn like some sort of resonance going through a hollow pipe. It lasts for a second or two and can be really loud. I can actually feel it through the gas or brake pedal. At first when my wife reported it (it's her car) I didn't believe her. She told me it happens when she slowed down to a stop or made a left turn. This past weekend we were driving into NYC from the Boston area and sure enough I got into the left lane to get on the highway and as soon as I came to a stop this really loud metallic horn sounding noise reverberated through the car. Scared the daylights out of me.
Well I got on the highway and didn't hear it for a good portion of the ride, was doing around 65-70mph. However, I started noticing that whenever I had to slow down I would hear it, although not as loud as when I was at a stop. So I driving along, lot of time to think about the causes and where it was coming from based on my wife's experiences and decided to shut off the AC and heater. Didn't hear it for the rest of the trip. Kept the AC/heater off for the whole trip back (250 miles) and didn't hear it once. I'm thinking it's comeing from the piping in AC system, maybe some valve sticking causing it but why only when I slow down or stop??
I am going to try to test this theory and reproduce it in my driveway this weekend but would love to know if anyone has any ideas.:jerking:

Figure an update would be good for anyone else who might have this issue. Decided it had to be the expansion valve making this strange noise so I changed the expansion valve and also put in a new dryer. charged the system and the noise has not returned and the AC is cooling nicely. Only issue now is sometimes the blend doors close while I'm driving and then don't get much air out of vents. If I turn the car off for several hours then the doors seem to operate normally. Not enough of an issue for me to bother with.