R129 aircon "crackles" when compressor cuts in

[Paul SL320]

[FONT=&quot]Hi,[/FONT]
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[FONT=&quot]This has been going on for more than a year now. A hiss followed by crackling occurs for a couple of seconds when the compressor is first switched on. [/FONT]
[FONT=&quot]I don’t mean when the compressor continues to run between cycles of low to high displacement. I mean when the evaporator temp drops down to 2°C and shuts down the compressor altogether, upon compressor re-start the noises occur.[/FONT]
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[FONT=&quot]The sound appears to come from inside the dashboard, near the central vent area.[/FONT]
[FONT=&quot]Since the problem started I've had the system vacuumed and refilled three times, by three aircon specialists, each weighing the contents of the fill to ensure correct quantity.
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[FONT=&quot]I've also had a new receiver/dryer fitted. [/FONT]
[FONT=&quot]The car has many vacuum controlled flaps in its ventilation system. After research on the system design I have been able to actuate each of these using the various buttons on the Control Unit. No actuator or combination of actuator movements triggers the noise.[/FONT]
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[FONT=&quot]On very cold days where the compressor never starts to run the noise never occurs.[/FONT]
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[FONT=&quot]The problem is caused by something related to the initial start-up of the compressor, but I don’t know enough about the system to work out what is the root cause, I can guess:[/FONT]
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[FONT=&quot]1.[/FONT][FONT=&quot] [/FONT][FONT=&quot]1) Expansion valve sticking allowing incorrect pressure control in the system [/FONT]
[FONT=&quot]2.[/FONT][FONT=&quot] [/FONT][FONT=&quot]2) Pressure signal hose to expansion valve is leaking or blocked[/FONT]
[FONT=&quot]3.[/FONT][FONT=&quot] 3) [/FONT][FONT=&quot]Compressor start-up is causing electrical interference[/FONT]
[FONT=&quot]4.[/FONT][FONT=&quot] 4) [/FONT][FONT=&quot]Compressor may be designed to start with gradual pressure build-up, and my compressor has developed a fault that means it starts at full displacement.[/FONT]
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[FONT=&quot]I would welcome your thoughts on what I could try next.[/FONT]
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[FONT=&quot]Regards[/FONT]
[FONT=&quot]Paul[/FONT]