Change in vehicle noise at 3100 RPM??

Around a year ago I posted a question regarding a LOUD droning noise that my new Jimny makes periodically, even after the keys were removed.  Its had its first annual service and the dealer's senior mechanic took it home (75km away) for weekend, but he was unable to replicate. They asked Suzuki factor and had no ideas. Its about to go in for 3 more days as its frustrating me no end.. 

Worried they wont be able to replicate it again I did some sleuthing this weekend and I think I worked out how to replicate it reliably. 

• While driving on freeway with RON91 and ~1/2 tank. 
• Outside temp 19c or 30c. 
• Drive at 3100RPM. Vehicle tone is to slightly louder around that RPM. Its like the reverberating noise large lorries/trucks make when they come off accelerator, but not particularly loud. Nothing out of the ordinary to my non-mechanic ears, but definitely deeper sound than at 2800 or 3300 (where normal engine whine predominates).  
• Noting this odd sound always happening at that revs, I tried holding revs for a few minutes to see what happens and bingo, each time the sound grew louder until it was uncomfortable.  All 4 attempts to trigger the sound were successful in two freeway runs. 
• When it has reached max loudness, I can do the following without the expected immediate drop in vehicle noise:
  • Clutch in and allow RPM to drop to idle, while driving at 100KPH. - ie sound is not engine rotation
  • Change up or down a gear - ie sound is not transmission rotation
  • Firmly press brakes - ie sound is not brake related
  • Very rapidly pull over and remove keys - ie its not road noise related. 
• Once the noise has reached max level, its sounds like a vacuum cleaner in the engine bay.
• Just as it gradually builds, it also fades gradually when the revs change by ~300RPM (up or down). The further from 3100 I go, the faster it fades, and the longer I am away from 3100 the longer it takes to build up again.  So in the above scenario of pulling over and removing keys, the sound fades over about 15 seconds after reducing below 3100, which is too soon to exit vehicle and inspect under hood.  

Finally I have a few good videos of this sound coming and going (inc with keys out) that Ill take to the dealer. 

The question for the forum is whether anyone else notices any change in vehicle tone around 3100RPM, whether or not it builds to a loud noise like mine?

Another question is what the hell could cause this. I can only think its air/vacuum/coolant pressure related given its still noticeable with keys out for ~10 seconds. Sound is coming from front of car. As its a droning noise its very hard to pinpoint it. Its definitely not the radiator fan or A/C by the way. 

Any suggestions would be appreciated.  
 

Just to clarify, it does this at 3,100rpm regardless of which gear you are in?

Yes, Definitely 4th and 5th. Cant recall if Ive tried 3rd yet.

And then once the noise is at its max, even taking out of gear and coasting only slowly causes the sound to fade away. ~10 seconds until completely imperceptible.

Its definitely coming from the engine bay? I have heard of speakers making weird noises due to interference from the alternator at particular RPM's, if the various parts of sound system aren't grounded properly. 
Similar idea to a PA system that will pick up and amplify radio waves through the speakers if the cable grounding isn't right. 

The only thing I know which will continue to run after the keys are removed on a standard unmodified car is the fan, they certainly sound like a vacuum cleaner and you mentioned the temperature of 19 to 30C which would suggest something getting hot. Even the radio switches off when you remove the keys.

However, you have already said it’s not the radiator fan.

Are there any other fans, perhaps one to cool the ECU or something else?

Does your Jimny have any modifications or after market parts?

Robin

Mmm, now thats a left field but weirdly logical possibility. Sound builds up and fades slowly because the electrical charge is being induced by alternator and then dissipated.

I think it is however coming from engine bay. When I jumped out once after slamming on brakes it was coming from there, but I couldn't pop the bonnet before completely faded.

PS there is also a fuel pump that momentarily runs after shut down, but thats a high pitched whinge for a few seconds after I get out of car. Im told thats normal, as its clearing the fuel line. 

The noise is so loud that it gives you a headache, inside the car. Hence why I need this fixed, as I will be unable to eventually resell the car as is.

Now that I can trigger it reliably Ill go back to square one and monitor the OBDII readings while its happening.

As for modifications, the first time I heard it was in the first week when I fitted a roof rack and took it out for the first freeway run. I initially thought it was some sort of harmonic wind effect on the roof rack. Really heavy cross wind that day. I have a few other minor customisations but all installed after the first time I heard the sound.

Actually, you just gave me an idea. If I can get it happening in 4th gear reliably, then I should pull over somewhere safe, pop bonnet and rev the car at 3100RPM and see if that makes it return. If it does, then it will be so much easier to fault than asking the mechanic to ride in the engine bay at 100KPH.

It still sounds like the cooling fan to me. How do you know it's not the fan, have you heard the noise with the bonnet up and seen the fan stationary?

I have just read that the power steering on the Gen 4 uses an electric pump (unlike the Gen 3 which uses a belt driven pump), not sure if it is true and I would doubt that would run with the keys out, but may be worth a look?

Robin

Most electric power steering uses a motor around the column, hence no pump or fluid.  There are electro-hydraulic systems but they're a bit niche.  It used to be thought impossible to make a 12V PAS system but technology moved on and killed off the proposed move to 42V cars, for which electric PAS was one of the main drivers.

I just had a look under the bonnet and there are no fluid hoses going to the steering box, so it must be the usual "dry" electric PAS.  Doesn't give feedback like hydraulic PAS, but Jimny steering is not exactly the pinnacle of feel in any of its guises!