Automatic levelling system?

Got a problem with my 95 blazer. When I start up the car in the cold i hear a loud, humming sound from the rear end. I had the same problem 2 years ago
and the shop replaced a compressor or pump that was supposed to level the truck with different load. I do not know the correct english word for this parts since I live i Norway (anyone who can give me the correct name for the part?)
They replaced this pump and the car has been fine since then but now the problems is back. The changed this thin on warranty and I will off course demand a new part. Just wanted to know if this is a well known problem and why this thin keeps failing!
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Do you tow ? It sounds like a fuel pump, but you say it levels the truck. Which means its some type of air compressor or hydralic pump. To me sounds like air-shocks or leveling bags. An some one put the pump in the rear.
Hope that helps.
Cheers

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I dont tow, only light use. The part is a air compressor or hydralic pump, and I am sure it is a standard feature on this model. I know the norwegian name for it but not the correct english one. I think it is supposed to lift the rear of the truck to keep til leveled under different load. .....
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Could the correct english word be "live rear axle"?
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You are right, it is a air compressor. It is rare for the compressor itself to go but I have seen it. Usually it is one of the plastic lines running from the unit to the rear shocks or bags that leak or the level sensor itself is faulty or disconnected. Turn off the truck, make sure it is in park and put on the emergency brake, also block the wheels if possible the crawl under the truck. Somewhere near the rear axle you will see a small box about 4 inches square and just under 2 inches thick. It will have a small wiring harness (about 5 wires) going to it. It will also have a lever. Attached to this lever is a small rod that connects to the rear axle. Make sure that rod is connected. Most often the problem will be that little rod comes off and the pump will run but the truck wont level. If all looks well then check the electrical connector at the sensor for corrosion. If all seems in order the a trip to the garage might be required.

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Thanks! since the shop allready has replaced the compressor once under warranty I will have them take a look at it.
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