Ford: HISSING NOISE IN AC, 2005 ford expedition eddie bauer, 2005 ford expedition

Question
HI. I HAVE A 2005 FORD EXPEDITION EDDIE BAUER. I'M EXPERICING A HISSING/LEAKING/PRESSURE RELEASING SOUND IN THE BLACK COMPARTMENT BOX ON THE PASSANGER SIDE UNDER DASH (ISN'T THIS THE EVAPORATOR?) AND TROUGHOUT THE REAR AC. THIS NOISE IS HEARD WHEN THE AC IS ON AND WORST WHEN THE FAN IS ON LOW. IT IS A CONSTANT CYCLE OF HISSING/LEAKING SOUND LASTING 10-15 SEC AND REPEATS EVERY 20-30 SECONDS - GETS LOUDER WHEN THE OUTSIDE TEMP GETS HOTTER 70-80*

IT IS UNDER WARRANTY BUT I CAN'T SEEM TO GET IT RESOLVED.
TOOK IT IN ON 4/16/07 AND WAS TOLD THAT IT CK'D OUT OK FOR LEAKS BUT WAS 1/2 LB LOW ON FREON. FREON WAS ADDED. THIS DID NOT STOP THE NOISE!!

RETURNED A WEEK LATER FOR SOMEONE TO DO A RIDE ALONG SO I COULD MAKE SURE THEY UNDERSTOOD THE PROBLEM. WAS TOLD NO NEED - THEY WERE JUST GOING TO ORDER A PART - AN EXPANSION VALVE.

RETURNED 4/30/07 - EXPANSION VALVE WAS REPLACED - THIS STILL DID NOT STOP THE PROBLEM.

MY UNDERSTANDING OF AN EXPANSION VALVE/ORIFICE TUBE IS THAT IT BASICALLY FILTERS ANY TRASH / DEBRIS ETC BEFORE ENTERING THE OTHER UNITS, AND IS LOCATED UNDER THE HOOD NOT INSIDE THE VEHICLE WHERE I'M HEARING THE NOISE.

I'M NOT HEARING NOISE IN THE FIREWALL THAT TRAVELS INSIDE. I CAN PHYSICALLY HEAR THE NOISE WHEN I PUT MY HEAD IN THE FLOORBOARD AND STICK MY EAR TO THE BLACK BOX. (IT'S NOT AS LOUD WHEN SITTING STILL) WHAT IS IN THAT BOX THAT I CAN HEAR PRESSURE BEING RELEASED? FAN? PRESURE VALVE? VACUMN LINE? SEAL LEAK?  COULD A PROBLEM WITH THE COIL/EVAPORATOR/CONDENSOR/BLOWER/ACCUMULATOR CAUSE THIS?
I LIVE 35 MILES-ONE WAY, FROM MY DEALERSHIP AND TRAVEL IS BECOMING COSTLY. PLEASE GIVE ME SOME TROUBLE SHOOTING INFO THAT I CAN INSIST ON HAVING CK'D TO RESLOVE THIS PROBLEM.
AT YOUR MERCY
MELISSA

Answer
Hmmm, Hi there Mellisa sorry to hear of your little nuisance. I think your dealership was on the right track with the expansion valve replacement. But did they replace the valve at the rear ac unit or just the one up front? The noise i have heard The noise will most likely be heard is when driving the vehicle with the interior cooled down, the rear A/C off, front A/C on, and the blower fan on the low speed setting. The big black box behind the glove box is your heater box. Now the noise you may be hearing could possibly be liquid freon (the product used to cool the passenger compartment) running through the evaporator core. This could happen due to a few reasons. Most commonly is the system has too much refrigerant, another is there could be air in the system with the freon. Now you are right with the orifice tube Mellisa, it does filter debris and particles from the air conditioning system. But it also creates a pressure difference between the high and low side of the air conditioner. with this pressure difference it allows the freon to boil and when it boils is gives us the cooling effect. I am not a molecular engineer to be able to go into anymore detail then that. I do think your dealership is on the right track though they just need to go a little deeper and verify the concern with you and retest after they repair it. If a vehicle is not repaired then you as a customer have a bad taste in your mouth.

Hope this helps

Jim