Unknown humming sound?

[mabryt]

Hi all,

Recently my '77 280SE, which is in otherwise excellent condition, started making a kind of deep humming noise when I accelerate at very low speed.  It's not a squeak, it's not a grind, just a hum that falls off when I go faster than about 15 mph.  The brake discs and rotors are a year old, so that's not it.  I checked the transmission fluid (which was flushed last year) and it seems fine.  It's definitely not coming from the engine, but somewhere else.

Any guesses, oh Wizards of the W116?

[jantar]

Its the fluid level in steering pump !

[mabryt]

Hi- thanks for the reply.  I am dubious, but will take a look and report back!

[mabryt]

Nope! The power steering fluid is just fine.  The humming sound is really more of a groan: it's inconsistent, but only occurs at low speed, i.e. 5-15 mph.  For any musical types, the pitch is not far from middle C.

[oscar]

Middle C, does it change infrequency at all or just fades in and out as you go faster?

Is there anything you can do to change the frequency or character of the hum - eg;
try reversing up to a decent speed,
go forward do some full lock u-turns,
any difference between accelerating hard and slow,
apply brakes with left foot whilst accelerating with right,

They may all be ifault free but I'm trying to think of ways to isolate the component/s making the sound so if you can think of anything else, try it.  If you could drive off in third gear I'd say try that too.

Check your tyres, check pressure and full rotational inspection especially the inner tread for uneven wear or missing tread.

Lastly, how could I forget.  Any chance that this sound could be uploaded as a sound file or youtube?  Some sounds aren't that noticeable when recorded but worth a try.

[mabryt]

Howdy Oscar- many thanks for the interest. 

The tone is constant and doesn't change frequency- it's just the volume that fades in and out.  It happens only when I accelerate slowly.  Haven't tried reversing or full lock u-turns, though: both interesting.   The tyres are only a year old and I just checked the pressure over the weekend- all fine.

Meanwhile, I'll try to get a recording.

Until then!

[Papalangi]

I thought I was the only one with a ghost under the hood.

It's a check valve for either the climate control or door locks.  Back off on the throttle and it will go away, only to come right back when you accelerate again.

My son calls it the whistle and asks me to make it do it again sometimes.

Michael

[oscar]

It's a check valve for either the climate control or door locks. 

Man I hope you're right, I know exactly what you mean.  Wish I thought of it.  
I replaced mine a long time ago during an attempt to find all the vac leaks in the lock system.  When the vacuum system was flat I'd hear a hum behind the dash as the vacuum built up but it was coming from the engine bay and I only realised the blue check valve was the culprit when I held it whilst the engine was warming up.  It was vibrating inside and was the cause of the hum.

[TJ 450]

My 79 450 has a high frequency buzzing sound that comes from the deceleration air valve at times... you can hear it clearly inside the car and you can feel the thing vibrating too.

Tim

[mabryt]

Well, darn.  It must be something else because it is NOT dependent on whether my foot is on the gas.  This morning when I left home there was no sound, but when I got to work about 30 minutes later, I noticed it as I let go of the brakes on a very, very slight incline and gently rolled forward in idle.  And it's definitely NOT a buzzing sound.  Maybe rather than a hum, it's more of a groan.  Hmm.

[MB_Mike]

I wanted to hold back on speaking up as I didn't think I had much to offer...I was (am) having the same noise but mine is accompanied by a rattle (I believe to be an unrelated heat shield). I think my groan comes from the exhaust resonator but had a MB mechanic look it over and not find any problems. He did however do the following work which resulted in a much more quiet groan as it was becoming quite unnerving:

Adjusted reverse band for transmission
Adjusted fuel mix
replaced fuel filter
checked timing

None of these would seem like they would have any effect but it seems like an awful coincidence.

Good luck and look forward to knowing how it works out.

[mabryt]

Hi Mike-

Thanks for the input.  I'll let you know what I discover.  Meanwhile, though, a question: what the heck is a reverse band (for transmission)?

[MB_Mike]

Hi Mike-

Thanks for the input.  I'll let you know what I discover.  Meanwhile, though, a question: what the heck is a reverse band (for transmission)?

Great question! My guy mentioned it was slipping in reverse (which I knew) and said he adjusted the band. I have heard that before but I couldn't explain if I had to. After I typed this I went to google...

http://www.familycar.com/transmission.htm#Transmission%20Components

[mabryt]

OK- here's the update.  First, the sound is becoming much more frequent.  Second, I realize that no matter what speed I'm moving, touching the gas pedal makes no difference at all - coasting or accelerating the sound is the same - BUT the sound STOPS if I touch the brakes.  The brake rotors and pads were actually replaced last year, though. 

Any ideas??? 

[oscar]

Are you sure it's not coming from under the hood?   My thinking is that maybe there's a vacuum leak in the Brake Booster and because of the way it's attached to the firewall it may resonate and sound like it's coming from elsewhere making it difficult to pinpoint the source.

The easiest and definitive way to check would be to have a mechanic apply a vacuum to the brake booster and check for integrity using a vacuum pump and gauge.  Also have him check the in line one way check valve for the brake booster whilst he's at it.

If you don't have the tools or don't want to take it to a mechanic yet you could try and make the hum happen whilst at a standstill.  After driving around and hearing it as often as you have, come to a halt then use the handbrake to hold the car steady then maybe rev the car to 2000rpm and back to idle to see if the hum still appear at any point.  I know you say it's not related to accleration which is fine, this is more of a test to change vacuum from the manifold.  The vacuum pressure will change with a change in rpm and engine load so if you don't hear it at idle, maybe you do when the engine increases rpm. 

Put the car in Drive with park brake still firmly on, try it again.  This will put a load on the engine and simulate better the vacuum the engine develops at the low speeds you hear the hum.   Note, cover the brake with your left foot because you may not get to 2000rpm before the park brake wont hold.

Whilst doing all this, if you can hear the hum whilst your stationary, apply the brakes with your left foot to see if that makes it go away.  If it does, and the brake booster is looking like it's the culprit, but you'll still need to have it vacuum tested to confirm.  Total cost for a new one and labour is expensive so you want to be sure.