Periodic loud droning at 100kmh

[GreyOne]

Hi there

While I was waiting to join here, I posted these questions on whirlpool forums with limited success. Im hoping you guys might have more insight.....

Can I get some advice on whats happening with my Australian Jimny GL/lite? The car is 6 months old and has 50 operating hours so Im reasonably familiar with it now. No modifications.

While freeway driving at 100KMH/3000RPM in 5th gear for ~30min, the car develops a LOUD droning noise, louder than the usual wind noise thru the roof rack, and loud enough to irritate the hearing. Over the wind, road and engine noise, this sounds a bit like a muffled car horn stuck on.

To cut a long story short, the hum:

* Started the first day I took it on the freeway (but I thought it was just the poor sound proofing)
* Comes and goes periodically without any change in speed. 5-10 minutes every once and a while.
* Is not altered by gear changes.
* Is not diminished by depressing the clutch and coasting (1000RPM) in neutral at 100KMH, at least not for 10 seconds.
* Strangest of all, it continues when the vehicle is rapidly stopped and the engine switched OFF, but conveniently drops to silence over a period of ~10 seconds. Due to the sound of the passing freeway traffic I couldn’t tell where the sound was coming from after racing around to pop the bonnet a few times. So it doesnt appear to be related to the engine rotation or drive train rotation. The reduction in sound after stopping and killing ignition is quite linear, like something is depressurizing or discharging. Vacuum pressurising?
Ill be taking it in for the Suzuki first service in 6 months, but any thoughts on whats going on here?

[jonfromhamilton]

Aircon?

[gwagensteve]

Sounds like the fuel tank spur purge to me. Only explanation for it to run after the engine has stopped.

[GreyOne]

Suzuki dealer mechanic had the car over weekend and put 100kms on the clock but couldn't hear the problem, which is what I feared would happen. However I was able to find some videos I had recorded of the noises coming and going. All agreed its a loud and abnormal noise.

Up shot is Suzuki agree it is pressure related. Likely either the Positive Crankcase Ventilation valve or the 4x4 vacuum system. They replaced the PCV and checked the vacuum system and car didn't make the noise after this (not that they could get it to make the noise before either).

So more test driving for me to see if its resolved. Ill be putting a microphone under the bonnet if it starts up again and see if this allows me to pin point the source of the noise.

Thanks for the suggestions.