Big_Taxi

HI, everyone. Going back through my old threads I alluded to this problem (and have thankfully worked out the others). This one is a doozy, and I keep forgetting to post about it because my Jeep's performance hasn't suffered, really.

I have a drone coming from under the hood/below my cassette deck/throughout my car, that gets LOUD (louder than my ambient radio) at 35-45mph and then fades to background until 60-70mph, then fades away to background again. This sound is unusual based on what I have looked at in the forums and elsewhere. It's annoying and worrisome, and the only clue I have (that I'm not sur eis accurate) is that it might have started after I removed and cleaned my throttle body... not sure though)

This sound IS: Droning, howling, moaning, monotone, middle-pitched (regular speech tone), ghostly, predictable at certain speeds, fairly loud (will get a dB meter after work today)
This sound is NOT: Grinding, whining, resonant, fluctuating in tone/pitch, hissing, stuttering, hacking/coughing, present when idling, a "whoosh"
This sound DOES NOT APPEAR TO BE: Related to RPMs/engine exertion (only mph)

I don't have performance issues, other than the engine feels "tight" when coasting down long hills, but the vehicle (Anthony) has had that since I have purchased him. I have gone on a few trips of 100+ miles and everything is pretty smooth, other than a shimmy ONLY through 50-55mph. Related? Who can say.

2000 XJ, 4.0 liter, 130k

Thanks for any help you can give, I really appreciate it.

EDIT: I'm going to try some suggestions that popped up, such as coasting in idle, and will update with results.

Big_Taxi

Loudest through about 0:40-0:50 ish

PatHenry

I notice that your RPM's do actually drop at that point in the video. I was going to suggest possibly tire noise, but considering your RPM drop that does seem to coincide with the odd noise, it seems to rule out strictly speed related possibilities.
Also considering the 2 ranges mentioned, would it be likely that it might be related to the specific gear in the transmission (2 and 4, but not 1 and 3 and with the assumption it's an automatic rather than manual)?

I'm not very familiar with automatics, but my first thought would be looking at the transmission related cable that is attached to the TB. Only because it's related to both the transmission and TB.

Edit - looking again it seems your tach is all over the place. I don't know if that's normal for automatic Jeeps, but for all the stick shift ones that I've driven it's obviously a steady RPM reading directly corresponding to the gas pedal. I would say it's definitely NOT something that happens with my other car (an automatic Corolla) the RPMs are generally very steady.

So I'd still consider that cable, I'd also want to make sure there isn't a very small leak in the TB area - cracked or broken vacuum line, bad O-ring on the IAC, improperly torqued mounting of the TB - that sort of stuff.

Big_Taxi

Oops, not sure how this works on mobile. RPMs are everywhere because this was taken through a few intersections after work. I do have a suspicion that it could be certain gears in the auto tranny, but have no real way of knowing without first asking. Also, I’ll update my post but it IS slightly resonant, through 45-35 mph, and doesn’t go away when in neutral, coasting.

The sound is as if you were to taunt a cow by moo-ing, but hold that M and slowly drop the note. I guess... so hard to explain.

PatHenry

Hmm... have you checked the tranny fluid condition and if it's not excellent, doing a drain and fill and noting if anything changes? It could also be some kind of wind noise - those are a pain to figure out but it can't hurt to go through and see if any and all plastic trim/mudguard stuff is tightly attached.

Big_Taxi

Wind might be a culprit, my mud guard is dangling so I'll tighten it up tonight and will be back with more info. Really though, the sound does have similarities to blowing over the top of a bottle, you know? Good catch.

Big_Taxi

Tranny fluid looks fine.

Big_Taxi

OK, I'm back. Everything's been same old-same-old, the sound is still there and right before NYS inspection I started getting a little idle shudder (and RPMs would go up and down when coasting downhill, so I figured I'd ask the shop to poke around. They checked everything I needed, but said that my throttle body/IACV had ...water? In the housing, or maybe tucked in near the valve itself? They drained it but I'm going to take a look and tighten everything up.

Also, there's a soft-ish plate/panel/shroud (what I called the mud guard earlier) under my engine that has a huge hole missing, that dangling part tore off. Source of the water? Source of the sound? How would I check, and is there a replacement part for this?

Thanks

firebane

This sounds like the plate to protect the torque converter and such and the large hole should be where the starter goes through... all speculation without pictures.

This would and could easily cause a rattle.

firebane

10char

dave1123

Have you adjusted the transmission throttle cable? Push the D-clip on the mount and push the cable sheave back as far as it will go. release the clip, get inside and push the throttle to the floor, listening for the sheave to ratchet out as the pedal reaches the floor. It will be harder to push it all the way down. This sets the proper length for the cable to control the shift points. Start it up and test drive it noting the shift points.

This may do nothing for your noise, but it will eliminate the transmission struggling to decide it's proper shifting steps. The AW4 transmission tends to have large rpm changes when in lower gears when power is on or off until the converter locks up. Then the rpm changes relative to vehicle speed just like a manual transmission until it unlocks. You can be coasting downhill at 40 mph and the tach reads 1500 when at 40 on the flat it may read 1800 or 2000 depending on weither it's in 4th or 3rd. The converter coupling seems to be a lot looser in the AW4 than in the 42re in my WJ anyhow. Same hill, if you drop out of O/D in my WJ, there is noticeable engine braking, but no so in my XJ. It's there, but you don't feel it as much.

That soft fiber panel under the radiator is a splash shield to keep water from splashing up into the engine compartment. Mine is barely still attached and partly missing, but I don't get your noise. It's not induction noise, is it? As in lack of intake tubing and filter? If the flywheel inspection plate is loose or missing, is something caught between the engine and converter that might be rubbing? Or something wrapped around the front driveshaft? On my XJ, the vent tube for the front differential got ripped off the diff and wrapped itself around the driveshaft. It wound up burning itself on the exhaust and stinking.

LGhigo

2000 Cherokee Sport 72,000 Miles, YES 72,000 Miles. This howling noise will occur Parked in my driveway, revving the engine to 1600-1700 RPM and also at about 2500-2600 RPM. It also occurs at different road speeds based on those same RPMs. I think it's in the exhaust system and I plan on attaching exhaust insulation to the muffler and Catalytic converter to see if that helps. It's a really strange sound that I never noticed in a '96 Cherokee 200,000 Miles that I had with same 4.0 and auto. Both vehicles 2WD.

dave1123

Have you checked your trans mount to see if the exhaust pipe isn't sitting in the crossmember and transmitting the sound up thru the body? Mine does this when I have to torque it up to start from a stop on a hill. I know that's what it is, but haven't fixed it yet.

BTW, my 2000 Sport only has 140K on it, but it was parked for a long time. 125K when I bought it last February.

LGhigo

Thanks for your answer Dave! Installed a Rough Country Steering Stabilizer today and while I was under there I rolled on back and checked the exhaust/crossmember interaction and it seems to be ok. I pecked on the muffler and catalytic converter and both of them seem to ring like a bell, so I am going to try some ways to damp that resonance. I will post what I find. Thanks again!