[Turbo]

It's not at all unheard of that "something" rattle only at a very specific RPM. Have to do with resonance frequency etc. Something I don't fully understand, but I've seen it proven.

Destroying vine glasses with the voice is probably the most famous example of that..

 

[i860]

Which is technically correct, the best KIND of correct .

That's why they, MB, created the M113k engine. Stronger parts, to take into account the added compression rate..

But just because something isn't designed for it, doesn't mean it's not a good idea.. Which have been proven .

On the lpg the problem could be the valve material ( but as we know MB made only good quality components see balance shaft, as intended of cours..) You burn the valves if you run lean, i you run lean lpg or petrol you have lambda probe to tell you that.
I will not go in any polemics but we are on a reputable forum and people are coming here for advices, poor or ritch ( as i was told long time by some one).
So your problem is not lpg related, slk 200, slk 280/350 all have faults, only bulletproof car is 55 less than transmision.

 

[Bohemian TopDown]

It's not at all unheard of that "something" rattle only at a very specific RPM. Have to do with resonance frequency etc. Something I don't fully understand, but I've seen it proven.

Destroying vine glasses with the voice is probably the most famous example of that..

Also true...
My wife broke some glasses yelling at me when I spilled a drop of oil in the garden, next time will measure the frequency.

 

[BaD_81]

Saying metallic noise, is it from the engine bay or under the car?
Gear Selector has a rubber, that I recently changed disgusting sound, that made me disassemble the cockpit more than once. If you are not sure what is checked, just raise the car on a platform/lift reach for the linkage between gearbox and shifter and make sure you shake it well. That bushing is 2GBP-2.5EUR let's say but the pleasure to replace it....

There is an example for the test, take the FUSE(s) of the lpg out - still the same - no LPG issue.
However inadequate LPG settings will lead to what i860 has covered.
SO, as a different approach, speak with a dyno test garage to check the car underneath (if possible of course) in 2500 rpms.
If they see a drop at 2500 rpms - that is serious.
If not, that would be every spinning component betweeen engine and exhaust - prop_shaft - has bearing, pulleys in the engine bay - bearings... they usually are audible in range, like 2000-2500 not only 2500, but who knows something very small could've keep exactly that frequency.
Can you make a video, like open your glove-box let the phone face up or down and start driving so the video gets that sound.
Someone can recognize it...

 

[efair]

@Bohemian TopDown I am just curious: is it easy to find LPG filling stations in your home town? I live in a fairly populous suburban area but the closest LPG filling station is a half-hour drive away.

 

[i860]

In Eu they have lots of lpg station, in my case i have one in my village, if that goes busted the next one is 25miles. So for 50mile go and back there is no point having lpg.

 

[Bohemian TopDown]

@Bohemian TopDown I am just curious: is it easy to find LPG filling stations in your home town? I live in a fairly populous suburban area but the closest LPG filling station is a half-hour drive away.

1 out of 3 gas station has lpg here, very easy to find

 

[Sneaky Pete]

As @i860 said lots of stations. In my small town in a 25km radius 63 stations.

 

[Bohemian TopDown]

Saying metallic noise, is it from the engine bay or under the car?
Gear Selector has a rubber, that I recently changed disgusting sound, that made me disassemble the cockpit more than once. If you are not sure what is checked, just raise the car on a platform/lift reach for the linkage between gearbox and shifter and make sure you shake it well. That bushing is 2GBP-2.5EUR let's say but the pleasure to replace it....

There is an example for the test, take the FUSE(s) of the lpg out - still the same - no LPG issue.
However inadequate LPG settings will lead to what i860 has covered.
SO, as a different approach, speak with a dyno test garage to check the car underneath (if possible of course) in 2500 rpms.
If they see a drop at 2500 rpms - that is serious.
If not, that would be every spinning component betweeen engine and exhaust - prop_shaft - has bearing, pulleys in the engine bay - bearings... they usually are audible in range, like 2000-2500 not only 2500, but who knows something very small could've keep exactly that frequency.
Can you make a video, like open your glove-box let the phone face up or down and start driving so the video gets that sound.
Someone can recognize it...

Thanks mate, tomorrow I'll make the video and post. Hard to say where it comes from, seems like somewhere between the engine and the cockpit, more to the right side.
When I test it in neutral, it still does the noise, but much less, so it's difficult to find the source.
And of course, when I am with the mechanic it's the quietest car in the world, a Tesla basically. Left the car repair shop, a tractor again.
In the next days I will also go and ask how much it would be to remove the lpg system, just out of curiosity. Again, I am happy with it, but every time you visit a mechanic they treat you like you have the plague. Liquid PlaGue.

 

[Bohemian TopDown]

As @i860 said lots of stations. In my small town in a 25km radius 63 stations.

Like, for real? Here in CZ I would say one every 10/15km.

 

[Sneaky Pete]

Like, for real? Here in CZ I would say one every 10/15km.

Yes for real.

 

[efair]

If the noise subsides when in neutral then I would suspect the motor/transmission mounts.

 

[BaD_81]

Thanks mate, tomorrow I'll make the video and post. Hard to say where it comes from, seems like somewhere between the engine and the cockpit, more to the right side.
When I test it in neutral, it still does the noise, but much less, so it's difficult to find the source.
And of course, when I am with the mechanic it's the quietest car in the world, a Tesla basically. Left the car repair shop, a tractor again.
In the next days I will also go and ask how much it would be to remove the lpg system, just out of curiosity. Again, I am happy with it, but every time you visit a mechanic they treat you like you have the plague. Liquid PlaGue.

I wouldn't be bothered to remove the Liquid PlaGue (some sense of you humor you have) just to prove it ain't it.
But it will not hurt to check the fittings/holders/clamps....etc.

 

[Bohemian TopDown]

Trying to upload the audio file...

 

[Turbo]

Upload it to YouTube and then post the link here..

 

[Bohemian TopDown]

Google Drive: Sign-in

Access Google Drive with a Google account (for personal use) or Google Workspace account (for business use).

drive.google.com

 

[Turbo]

Yeah, it just asked me for access.. You need to make the link public..

 

[Bohemian TopDown]

Should work now...

 

[Turbo]

Doesn't sound like a bracket or flap or anything, sounds more like a metal shielding being loose somewhere..

There's one, at least on my 55, between the engine and the firewall, but also a few underneath between the exhaust and the chassis.. I'd check those first.

 

[efair]

Nobody is gonna listen if you post a link that asks for a password...