Moaning noise, Gilly, MB Doc?

[Toad]

First off, I want to thank Gilly, MB Doc and everyone else who contributes here. There is wealth of information to be found. I went back and forth on whether to by and ML or an RX300 for my wife. I was a BMW mechanic for nearly 20 years and worked on many Porches and MBZ's too, but had heard a lot of negative stuff about the ML's. The info available here and the exceptionaly clean ML I found swayed me over to the purchase.

We now have a very clean '99 ML320 with only 39K on the clock. We've had it since December 26 and so far so good. (Dealer Starmark Purchase)

It just developed an odd "moaning" noise last night. The sound is very intermittent and lasts only about 20 seconds. It is not cyclic, or speed related. Downshifting, speeding up or slowing down do not effect it. It is hard to pinpoint because it lasts such a short time. It sounds as if it is either coming out of the HVAC area around the passenger side, or in the engine compartment in the area of the Aux coolant pump. It's actually emminating more in the area directly below the windshield on the passenger side. I am not yet familiar with the ML's HVAC system, but it sounds a lot like a lazy vacuum motor.

In any case, this started right after some recent work by the Dealer and myself:

The Dealer did a transmission fluid-filter service, both F/R diff oil service, transfer case oil service and the updated fuel filter with a new pump and fuel level sensor. They also installed the updated sunroof insulation panels.

Later the same day after bringing it home, I installed new spark plugs, a new air filter and a new cabin air filter. I installed the air filter with the short step toward the front of the vehicle so it doesn't block the intake snorkel and the cabin air filter as it came out with the foam edge-wrap toward the passenger door.

Any ideas?

Many thanks,
Toad

[Toad]

Forgot to add........the Dealer also enabled the AAM for the fog lights. Can't see as this would have any bearing unless something got knocked while they were in there.

Toad

[Gilly]

I sure can't think of anything. Will the truck continue to make the noise at a standstill, maybe you can track it down that way (stop truck and go over and listen).
Once in awhile you'll see the blower fan collect leaves or stuff like that and it'll make a moaning noise, but it'll do it pretty consistently.

Gilly

[Toad]

Thanks for the reply Gilly. That is exactly what I need to happen to find it. The first time I heard it was just as I was closing the hood...and it stopped by the time I opened it back up. The Wife heard once on her way to work, once on her way home--about 20 miles each way. I went for ride last night and it made it once after about 10 miles of trying different speeds, different gears, monkeying with the heater controls etc. Nothing in particular seems to trigger it. It's a bit like resonance of some sort......like a valve opening and something bleeding off, or a vac motor, or HVAC flap actuator.....just an odd sort of slow moan that tapers off after about 20 seconds--very odd.

I'll just have to make time to fire it up and run it for however long it takes to track it down.

But, on a good note, I always liked moaners anyway!

Toad

[Gilly]

He he he, yah.

One other thought is possibly the aux water pump. That little baby runs whenever the engine runs, plus about 5 minutes after shutdown, you've probably noticed it running in your garage assuming that's where you park. Maybe that thing is intermittently noisy? It's on the engine side of the sheetmetal shield around the battery, you'll see the heater hoses running to it. You could probably safely remove the wiring connector on the motor as a test to see if the noise goes away.

Gilly

[Toad]

Actually Gilly, you may be on to something here. When I changed out the cabin air filter there were quite a few leaves in there. I'm guessing it had not ever been changed. Perhaps when I swapped it out a few leaves fell over toward the blower motor. I'll have another look in there.

Thanks again,
Toad

[Toad]

Yes it could be the aux coolant pump too. It has made only the usual buzzy noise so far as I've heard, but I'll take a closer listen.

Toad

[Gilly]

On the blower fan, you'd think if that was the problem, it would either be a problem or it wouldn't-not come and go, plus it would be more fan-speed dependent.
The aux water pump problem may come and go.

Gilly

[Toad]

Yes, I agree. Given the approximate origin and tone of the sound, the Aux pump really does make the most sense. Time will tell!

Toad

[dawhiff]

I had the same thing. There may be a diaphragm or valve in the pump. I tapped mine with the handle of a wooden screwdriver (high tech) and it stopped and has never done it again. the heater pump is on the shotgun side next to and below the battery

[Toad]

Thanks Dawhiff,
I wish mine would just make the noise long enough for me to actually pin-point it! It will sometimes go a week without a peep. When it does happen, it stops before you can get the hood up.

Maybe it needs a little Luv-Tap too!

Toad