After our air conditioner runs a couple of times we hear this huge violent noise for 1 or 2 seconds. Every time the outside air conditioner turns on, then about 30 seconds later - there is a loud, violent noise and then it runs smoothly for the rest of the cycle. Does anyone know what it could be? Robert

It is one of a few possible things.
1. The contactor is going bad and the unit is losing power on and off until the points on the contactor "make".
2. The system is getting refrigerant "migrating" to the compressor during shut down. This means that the shell of the compressor has liquid refrigerant in it and when that get in the compressors pump it jumps and goes wild. Very dangerous to the life of your unit.
3. Most common is there is something in the way of the fan blade on start-up. When the fan gets up to speed it moves or gets hit out of the way.
A little more observation and details on the sound would help me get this closer.
Look at it and find exactly where the noise is coming from. Describe the noise (grinding, shaking, knocking).
There are no Gremlins and you my save your unit.

It is one of a few possible things.
1. The contactor is going bad and the unit is losing power on and off until the points on the contactor "make".
2. The system is getting refrigerant "migrating" to the compressor during shut down. This means that the shell of the compressor has liquid refrigerant in it and when that get in the compressors pump it jumps and goes wild. Very dangerous to the life of your unit.
3. Most common is there is something in the way of the fan blade on start-up. When the fan gets up to speed it moves or gets hit out of the way.
A little more observation and details on the sound would help me get this closer.
Look at it and find exactly where the noise is coming from. Describe the noise (grinding, shaking, knocking).
There are no Gremlins and you my save your unit.

Thanks for your input Mr. Air. Today I have observed the outside air conditioner and learned the following:
1. The noise is not happening at all.
2. The difference today compared to other days are:
a. the outside temp is only 84.
b. the thermostat is set at 78 instead of 79
c. the air conditioner turns on about 5 times per hour and runs for very short periods. ( a lot shorter than usual).
d. the air coming out the top of the air conditioner is not hot like usual, and the sides of the air conditioner are not very hot either like usual.

To give more info on the noise: after 30 seconds the air conditioner vibrates real loud (scary loud) and sounds like it may blow up. I think the noise comes from the bottom part of the air conditioner.

Thanks for your input Mr. Air. Today I have observed the outside air conditioner and learned the following:
1. The noise is not happening at all.
2. The difference today compared to other days are:
a. the outside temp is only 84.
b. the thermostat is set at 78 instead of 79
c. the air conditioner turns on about 5 times per hour and runs for very short periods of time. ( a lot shorter than usual).
d. the air coming out the top of the air conditioner is not hot like usual, and the sides of the air conditioner are not very hot either like usual.

To give more info on the noise: after 30 seconds the air conditioner vibrates real loud (scary loud) and sounds like it may blow up. I think the noise comes from the bottom part of the air conditioner.

I guess I spoke too soon. The sound just went off 10 minutes after the last email. It was a lesser sound, but still making the sound to a lesser degree. What do you think?

It really sounds like refrigerant migration. This is when in the OFF mode, between cycles, the refrigerant fills up the compressor with some liquid phase of itself. What I mean by that is normally in an a/c system there is liquid and gas. The compressor can only handle gas. So what is wrong? Well the liquid phase of the refrigerant is supposed to be on the high side of the system. This is the condenser's coils, the small copper line into the house (high side) and up to the entrance of the evaporator. Just past this point is some device that converts the liquid to a gas (this is how "cooling" happens, I'm keeping it simple). In your case the attic is so hot that the refrigerant in the off mode "migrates" or moves some of its liquid to the low side and out to the beginning of the condenser, that being the shell of the compressor.
Your attic is hotter than outside or just the way your system is piped.
How to stop. Two ways.
1. install a high-side solenoid valve. When the unit turns off the valve closes and clunk no high side goes past this valve. It is put at the evaporator input and wired to the 24volts to the condenser.
2. install a compressor heater. Simple, it keeps the liquid from finding a "home" in the off mode. All compressor used to have them, but that was before we became a throw-away society. LOL

Not to scare you but this kind of thing if I am right can destroy your compressor. So you are right to worry about it. There are other ways to prevent this but I wanted to give you the most common ways it gets solved and not to be confusing and seem esoteric . Dave