I’ve got a quirky problem with my Mercedes that, while not affecting performance, certaily is somewhat bothersome.
It’s an '88 convertible and whenever the hardtop is on, everything’s fine but if the top is off or just the ragtop is up it makes this noise…
It’s a humming sound that circulates in both intensity and pitch on about an 8 second interval. It starts off low and soft (hmmmmm) then gradually builds up louder (hmmmnnnnnNNN) and then trails off again (hmmmmnnnnnNNNnnnmmmmhh) only to repeat itself every 8 to 10 seconds.
It’s coming from the front of the car but difficult to discern if it’s from under the hood or not. I can’t tell if it’s a mechanical part or if air is building up and then bleeding off from some chamber.
It sounds like two frequencies are causing a “beating” phenomenon. This is due to interference between the waveforms of the two frequencies. It could be that the frequency at which the ragtop is vibrating is almost the same as the frequency of the engine, in which case you’d hear the beating.
I can’t explain why you’d get this if the top was down, though.
Definitely sounds like it could be a resonance thing of some sort.
Just to confirm, it’s an '88 560 SL, with the unpowered ragtop, right?
It’s so unlikely that this is the solution that I considered not mentioning it. However, I did once fix a strangely similar noise problem on an SL, so there’s the snowball’s chance…
Next time you can, take a close look at the grilled air intakes just in front of the windshield. The passenger side one is plainly visible here on this '86. Here is a shot of the engine bay.
On the car I was dealing with, a bunch of sycamore seeds had managed to get wedged in the air intakes and in the springs of the hood hinge, which you can see in the second picture. When the car got going fast enough, they would start to make a buzzing sound. You may be suprised at how much debris finds its way into the back corners of the engine bay of that particular model. The car I was working on had to be blown out with compressed air. I suppose its possible that having the hardtop on either dampens the noise or changes the airflow under the hood.
Anyway, good luck and if I managed to get that one right, it’ll be near-miraculous. Very, very nice car, by the way.
I just had another thought. The two frequencies involved could be the engine speed and the natural resonant frequency of the car itself. That way, it would happen whether the ragtop was up or down. The reason it doesn’t happen when the hard top is on is that the hard top provides some measure of additional structural rigidity to the car, changing its natural resonant frequency. With the hardtop gone, the netural resonant frequency of the car may drop to within a few Hz of the engine speed, producing the beating effect.
Is it really coming from the front of the car, or does it just sound louder when you lean forward? Also, as Nevarmore hinted at, if the above is happening, it would only do so at specific speeds. Does it happen no matter how fast you’re going (or how fast the motor is revving)?
NevarMore, I don’t think it’s a ragtop issue, as it happens with the rag tucked safely away as well. In fact, I misspoke earlier and it does happen with the hardtop on but the sound is surpressed enough in volume to really be pretty much unnoticable.
And Syzygy, speed only seems to affect to volume of the humming, not the pitch or the frequency.
Sofa King, are you psychic? Your information, though not provided, was quite accurate.
I’ll check the intakes and any other cavities near the back of the engine compartment to see if “vibratory material” has found a comfortable resting place.
Thank you all, I’ll check on your excellent suggestions tonite!
Syzygy, if you’re correct on the battling harmonics, shouldn’t I be able to test this by dropping down a gear or otherwise changing my rpm in the middle of an episode to see if this alters the pitch or cadence? Thanks in advance.
Yes, that would certainly do it, but if it occurs at different speeds, it does so with two different engine speeds. Thus, it seems not to be a resonance problem. It just sounded like one by your description of the “beating” effect.
In lieu of the resonance explanation, I’m leaning more towards the “pressure buildup and release” explanation in your OP. I’ll leave it to a mechanic to decide where that could be happening.
