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?