Chrysler Repair: General A/C question, vacuum hoses, jeep cherokee

Question
General A/C question. '92 Jeep Cherokee. When air is on there is an intermitent LOUD "eeerrrr". Thought it was blower motor going out - changed - not it.
It seems like it is in the system. The lines going into the firewall vibrate when this is happening. Would being low in coolent or ...? cause this sound?

Answer
Hi Steven,
Unfortunately I don't have experience with the Jeep heater/AC system so I can't offer much solid advice. There is the evaporator unit where the cooling occurs and perhaps its orifice is the source of the noise when the refrigerant expands but I don't have any experience like that which you describe to fall back on. The only other moving parts have to do with the air control doors that may be vacuum or electricallly operated but I don't know which in the Jeep system. If by vacuum, sometimes a crack in such a hose will cause a whistle. If it were vacuum operated there would be a vacuum supply line probably branching off the power brake canister that also goes thru the firewall. You could disconnect the line and plug its connector on the manifold/brake booster side, which would shut down the door system of course but it might then no longer make the noise which would be a clue to look into the vacuum hoses on the cabin side of the system. I wish I could be or more help but that is about all I can think of.
Roland