High pitched ring/whine at low speed

APC9199 · **Posted:** Wed Apr 05, 2017 11:06 pm

I've noticed a strange noise from under the hood when I decelerate. It is worst around 10-15mph and slowly increases in pitch until I stop and it disappears. I can't find it because I can't run fast enough, long enough, to let the Jeep roll down the road while I listen for where it is coming from :ROTFL:

It honestly sounds like the noise a jet engine makes when it rev's up on the tarmac, just a tiny version of one!

It doesn't really sound like a vacuum leak or a boost leak, but then again these Jeeps don't ever act like you would expect them to. I have IDParts CAC hoses which have been spray sealed to the hose fittings and the air box to turbo hose is in good condition. I'm not sure how I would inspect them for leaks when none are obvious at a stand-still. The hose pressures should be low then anyway. Is it possible for a solenoid to make a noise like that? I know they can make a buzzing noise, but this is more like a constantly increasing tuning fork noise. How do I start tracking this down???

flash7210 · **Posted:** Thu Apr 06, 2017 9:49 am

Is the sound related to engine speed?
Like, you can hear it while reving in neutral?
If so, it might be a bad pulley.

Or is it related to wheel speed?
If so, it might be a wheel bearing or dragging brakes.
Or a bad front driveshaft joint.

APC9199 · **Posted:** Thu Apr 06, 2017 11:48 am

It doesn't sound mechanical, really. It only happens when slowing down or while coasting at very low speed. At 10-15mph it sounds like a small jet engine or a dentist drill, a high pitched whine. As I slow down even more it increases in pitch until I am at a complete stop and it goes away. It isn't related to wheel or engine speed, it only happens when slowing down at very low speed. It also isn't brake related. I'm not sure if I can record it or not, but I can try. I'm thinking it must be something to do with the turbo/vacuum system but I don't know what to check to see if I am right.

flash7210 · **Posted:** Thu Apr 06, 2017 12:20 pm

If it were a vacuum problem, your turbo wouldn't function properly (low boost) and you would notice a stiff brake pedal (vacuum assist). But it cant hurt to inspect all your vacuum lines.
When slowing down at 10-15 mph the turbo is generating very little boost and should be winding down (lower pitch).

Other than a driveline problem (bearings, CV joints, diff gears), I've got nuthin.

GordnadoCRD · **Posted:** Thu Apr 06, 2017 1:32 pm

If it were vacuum leak related it wouldn't stop when the vehicle stops. Even a leaky brake booster would continue the sound after the vehicle stops.

I'd look again at your brakes. To me it sounds like a bit of gravel caught up somewhere and is trapped against a rotor, possibly even in the rear.

Have you checked both drive lines? You could also have a stick or something wedged up against a prop shaft, or even a front axle. If you have skid plates they could conceal something caught in there.

What you're looking for is something that stops moving when the vehicle stops.

rankom · **Posted:** Thu Apr 06, 2017 4:59 pm

also check exhaust manifold gaskets for leaks ?

olypopper · **Posted:** Fri Apr 07, 2017 9:52 am

My Jeep strange noise turned out to be the mechanical fan clutch......and behaved very much like yours.

joe_ · **Posted:** Fri Apr 07, 2017 12:52 pm

In order to determine whether the noise is coming from the engine or further down the drivetrain, go to a big empty parking lot, put it in Neutral while driving ~15mph, shut off the engine, and see if the noise stops with the engine or keeps on going as long as the vehicle is moving.

APC9199 · **Posted:** Mon Apr 10, 2017 11:43 am

joe_ wrote:

In order to determine whether the noise is coming from the engine or further down the drivetrain, go to a big empty parking lot, put it in Neutral while driving ~15mph, shut off the engine, and see if the noise stops with the engine or keeps on going as long as the vehicle is moving.

That's a good idea! After listening to it some more, I'm positive it is more engine related than drivetrain related. It doesn't seem to necessarily follow the engine RPM or Turbo spool directly, but it varies more with the engine load than with the wheel speed for sure. I've also noticed that it does happen at higher speeds, upward of 35 or 40 mph, but it is significantly harder to hear due to engine and road noise. And again, it is very much NON mechanical in nature. It doesn't sound at all like a grinding or interference noise. It is the type of noise that you almost cant hear because it is so high pitched. I've had most of the front and rear end apart doing bearings and ball joints recently, so I would have noticed any rubbing/wear spots or sticks, etc.

GordnadoCRD · **Posted:** Mon Apr 10, 2017 5:16 pm

That, quite honestly, sounds like normal turbo compressor noise.

Have you recently changed your air filter? Sometimes that can change pitch and volume enough that it's noticeably different than what you're accustomed to.

APC9199 · **Posted:** Tue Apr 11, 2017 5:02 pm

GordnadoCRD wrote:

That, quite honestly, sounds like normal turbo compressor noise.

Have you recently changed your air filter? Sometimes that can change pitch and volume enough that it's noticeably different than what you're accustomed to.

I'm due for an oil change, so I'll do the air filter again and see what happens. They don't last long here in Arizona with all the dust and sand!

High pitched ring/whine at low speed

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