I have a 14 plate Fiesta - the air conditioner in the car seems to be linked to the main air intake so if you turn the aircon on it sounds like the fan is working really hard (which I guess is because it is).

Anyway, yesterday I noticed that when I turn the engine off there is this low rumbling vibration - it sounds like there is a large truck passing you and making the ground rumble. I tested it twice today, one half of the journey I had the A/C on then pulled over and turned the engine off and it rumbled, then I did the rest of the journey with the A/C off and when I turned it off, no rumble.

Weirdly if I turn the engine back on and wait for about 2 or 3 mins with the A/C on then turn it off there is no rumble so it seems to be with the A/C on with the engine running for a while?

I have noticed that the air hasn't been really cold over the past few weeks and interestingly thought I should run it in for a re-gas. Would a lack of gas cause this shudder / rumbling noise?

  • Welcome to the site. Hard for us to know exactly what is causing the rumble you describe. It sounds like you have done a good job narrowing it down to the AC system. As far as the lack of refrigerant causing it, it is possible that the reason the refrigerant is low or gone could certainly point to what may be causing the issue; i.e. bad compressor. I would suggest taking it in for a service.
    – CharlieRB
    Sep 21, 2017 at 11:51
  • Hi, apologies for sounding dumb here but would a bad compressor cause the refrigerant to leak or become lower?
    – Connor
    Sep 21, 2017 at 15:30
  • Yes, it is possible.
    – CharlieRB
    Sep 21, 2017 at 17:29
  • @Connor I’ll second taking it in for service. The only mechanism that could be making sounds like that would be the compressor, but that should be shut off when the engine isn’t running. It’s an odd case, and I say the best way to get a good answer is to have a competent yech look at it directly. Nov 20, 2017 at 19:24
  • Also, in many cases the system is lubricated with oil added to the gas lines, so a leak can also result in insufficient lubrication which will burn out the compressor and cause all sorts of rough operation. Nov 20, 2017 at 19:25

2 Answers 2


A/c condencer fan may be faulty When its speed decreased to stop Due to turn off ignition


A clutch equipped AC compressor like the one on your car isn't pumping whenever the engine or the AC system is off (and, most importantly, whenever the clutch coil isn't engaged so that the clutch hub isn't rotating).

The system equalizes when turned off, that is, refrigerant where the compressor once kept a high pressure starts migrating wherever the pressure is lower once the compressor is turned on. All of this unless the system features a liquid line solenoid valve that closes once the compressor turns off, which small AC systems such as car ones usually don't feature.

The noise you hear might just be the AC system equalizing.

A properly done AC system check, evacuation, vacuum and recharge cycle, especially if you ask the AC professional how much refrigerant has been pulled from the system, would surely do good to the AC system, and it may also help pinpoint any eventual leak. In addition, the compressor's oil usually comes diluted with a dye that shines whenever it's exposed to blacklight.

  • I'm thinking the same thing, hearing it pass through the orifice/expansion block although I've never heard anyone describe it as a truck passing which sounds like it could be something wrong with the condenser fan if it's separate.
    – Derek
    Mar 21, 2018 at 22:52
  • It has to be an H block type internally equalized expansion valve. Orifice tubes are obsolete technology in car ac systems. Definitely has to check if the condenser fan stays engaged when the AC system is turned off and he hears the sound he describes that way.
    – Al_
    Mar 21, 2018 at 22:58

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