tgbarton

I recently purchased a 1991 300SL. There is a loud droning noise - the kind that comes with harmonic resonance - when driving between 50-60 mph, but only when I have the hardtop on. It has nothing to do with RPMs, whether or not the engine is warmed up, etc. The hardtop is solid and well-sealed and pushing up on it has no effect on the noise. Does anyone have an idea what could be causing it and how to fix it?

MB-Dude

Question: If you 'crack' both the windows down just a bit while driving, does the sensation go away or subside?
Cheers,
Jeff

tgbarton

Thanks for the suggestion, MB-Dude, but so far no go.

Cracking the windows and/or playing with the air circulation didn't have an effect. I also tried putting it in neutral to see if that stopped the sound, but it didn't, nor does weaving back and forth change the the tone/volume at all. There's no shimmying or unusual tire wear and the alignment on all four tires checks out fine. When changing speed, the sound only starts up at about 50mph, peaks at around 53mph and tapers off in volume by 57mph.

MB-Dude

By your initial description, I would have suspected oscillating air pressure in the cabin - especially with regard to the hardtop. But if cracking the windows a little doesn't change it, then the 'droning' is probably elsewhere.

Nevertheless, try driving when the drone is the loudest and turn the AC unit to 'Recirc'. Make sure you can hear the baffles changing position. That action should cease to allow air to rush into the cabin from over the cowl.

Assuming that doesn't have an effect, do you feel *any* sort of minor vibration in the steering wheel, especially at the infamous 53 MPH point? Wondering if maybe a front wheel bearing is getting worn. Or perhaps a rear wheel bearing, though when one of those starts to go you sense it more in a particular turn direction.

If it's not engine speed related or cabin pressure, there are only a couple moving parts relative to street speed - wheel bearings, rear axle gears, driveline. Though maybe you have something loose underneath the vehicle and it vibrates at a particular frequency, at a particular speed. Check the 'belly pan' under the engine; it's held in pace with 4 screws that become stripped over time. Short of actually hearing the noise, it's tough to second guess the issue.

Cheers,
Jeff

tgbarton

Alas, moving the vents to re-circ didn't change anything. However, yesterday was a windy day and I noticed that the noise started at a lower speed when heading into the wind. That seems to indicate the "something loose" hypothesis, so I'll start checking around underneath. Thanks again for the help!

Floobydust

If you can hear the noise more than feel it, check the outside metal trim pieces that cover the rubber seals on the rear and side windows of the hardtop itself. A loose piece of trim can form a spring that can vibrate in the wind, a phenomenon known as aeolian vibration. Objects that undergo aeolian vibration tend to exhibit a very high "Q" which results in a narrow range of excitation conditions that cause it to happen. The narrow speed range in which the noise occurs seems to fit this behavior.

To check for this, cover all these trim pieces with masking tape and go for a drive. If the noise goes away or significantly changes, you have found your source. Start peeling off tape until it comes back, and now you have your loose trim piece.

If the noise is more felt than heard, check or have someone check the flex disks on the drive shaft. Worn flex disks can cause vibrations in the 40 - 60 MPH range. Common problem on the R129 and W124.

Hope this helps (I hate "noises" too).

- FD