Jeep question -- This one has me stumped

[HawkeyeScott]

A co-worker of mine drives a 2001 or 2002 Jeep GC, 4.0. She has what I describe as a whirrrrr-like sound coming from the lower parts of the dash area, in the center. I have never heard anything like it. And whatever it is, it is not impacting the performance of the car. Here's what I can tell you about it:

- It becomes audible at about 25 mph, but is very noticeable at 50 mph and up.
- The noise is ONLY made while the accelerator is pressed. As soon as she lets off, the sound goes away.
- When she lets off, it almost sounds like a valve is closing. There's a quick flap or burp sound, and then the noise stops. As soon as she's back on the gas, the noise starts.
- It makes the noise whether the A/C or heat or fan are on or off. I shut them all off and it made no difference.
- The Jeep is running and performing fine otherwise.
- She did have the Jeep's transmission flushed and oil changed before the time she thinks it started. (But you know women and how observant they are when it comes to their cars.)

I think that's it. Any ideas? I'll be anxious to hear what you think.

 

[Gary Allan]

Exhaust manifold cracked ..possibly. It would fit the inciting components to the problem ..if not fit exactly the noise description.

Some suggestive isolation type diagnostic techniques (other will add more, I'm sure).

You didn't mention if the noise presented itself in PARK while revving the engine or ..with brakes firmly applied ..if you could get it to do it in DRIVE with raising the idle toward converter stall (sensible caution advised).

Get in the speed zone that the noise is most evident ..let's say 50 mph (you may need to do this when traffic will be light and permit some latitude in self indulgence). Put trans in NEUTRAL and allow engine to retreat to idle. Rev the engine. Does the noise return? If yes, the condition should be isolated to the engine (and I'd look to the exhaust manifold as a possibilty). If not, then the flexplate back is where to continue your search ..typically anyway (there may be other obscure possibilities that others will add).

Since you didn't report any apparent vibration and/or cyclical harmonic component to it ..I have, for the time being, excluded motor/trans/tcase mounts and related hardware that could result in some contact or misalignment issues while under load.

 

[Greaser]

Make sure that her T case is in the 2W hi position and not lingering in between 2w hi-4w hi mode,I.ve seen this happen when the shift lever for the 4x4 wasn't fully frontwards..Check to make sure when they did the transmission flush somebody didn't get a transmission line around something it's not supposed to be around...like a breather tube or linkage down there.

 

[TedT]

Could it be that the transmission fluid is low allowing air to mix with the fluid? This will cause a whirrring noise.
Ted

 

[HawkeyeScott]

Since she noticed it after the transmission flush, I immediately thought it might have something to do with an incorrect fluid level. I checked (when cold), and it was well above the area the manual states it should be. Can't believe I forgot to mention that.

Any chance there is TOO MUCH fluid and it's somehow causing the whirrrr sound?

(The oil level is also higher than I think it should be. Maybe she had the work done by a chronic over-filler.)

 

[TedT]

You bet it could. Rotating parts passing through too high fluid at road speed will whip the fluid into a foam. The air in the foam going through the pump causes the noise.
Ted

 

[TwoBills]

You have to check the transmission fluid level w/the vehicle at operating temp.

 

[HawkeyeScott]

It was above the area on the dipstick the manual says where it should be when checking it "cold" (70F).

It was actually above the "hot" area too. Perhaps I will tell her to go have it drained, counted and then refilled. Hopefully it's that simple.