w123 om602 massive "humming" noise.
 

I can start of by apologizing that my english aint to good so i might have some issues describing my problem.

I where the one that swapped from a 300d (om617) to om602 and 5speed gearbox and the noise where there with the old engine aswell.

Im having a issue with my old w123, basically when im driving between 50-75kmh (30-50mph) im getting a LOUD humming noise that transmits through the whole body of the car (almost like a exhaust laying against the floor). It's almost impossible to tell what end of the car the noise comes from.
The noise is there in 3-4-5gear when the engine produce as much torque as possible, if i put the clutch down in those speeds the noise goes away.
The only way to get thru that speed without noise is by going full out in second gear until 40-45mph and chaging to third so the engine wont produce any torque.
I have driven 90mph with it and the noise never comes back when i come over 50mph.

I changed the engine mounts, transmission mount, subframe bushings, both flex discs (Lemförder), center support bearing and mount, u-joint, driveshafts and propshaft bushings.
Differential mount is ok and wheel bearings is ok.
I tried a different propshaft aswell but i had the same issue with that one.

I really cant figure out what's causing this issue and neither can any other people i speak with so now i turn my question towards this forum.

Regards from sweden

I like the concept. How fast is it and what fuel consumption are you getting? how hard was the swap?

Have you checked to see if your exhaust is making contact with the body somewhere?

Im not really sure yet, the car is undriveble with this noise.
Swap was pretty easy.

Ofcourse i checked it, the exhaust is nowhere close to hitting anywhere.

Flywheel balance? I'm short on ideas at the moment.

I get a hum similar to what you've described under similar circumstances. It's been there since the manual transmission went in. I've never done any troubleshooting on it, since mine isn't very loud and I kind of like it. I've long suspected it has something to do with the fact that I'm running a 616 flywheel on a 617.

Could be that the 2 piece driveshaft was put back together improperly. With these cars, and some other cars (volvo 240 comes to mind) you must mark the 2 pieces of the driveshaft before disassembly, if you don't then you'll have a large chance of reconnecting it in different splines, and the balance will be off just enough to bother you. Same thing happened with my first w123 when I did a motor swap back in 2009. I was just talking to a local mechanic about the same thing last week, and he's had it happen twice on customer cars as well.

Uhm i never even thought about that but im higly sceptical that an unbalanced flywheel would cause a noise only at those speeds.

I've tried multiple propshafts and the problem is the same all the time, doesent matter what i do to it and it's straight as a whissle (0.2mm play at most).
Propshafts usually makes noise at higher speed aswell if im going by earlier cars i've had with unbalanced propshafts.

But when you are swapping these driveshafts, are you marking them prior to removal from the vehicle they come out of? They must be put together exactly how they were, otherwise vibration nation happens.

Yes ofcourse they are put together as they are supposed to be.
I even tried to unbalance it to see what would happen by putting hoseclamps on it and then i got vibration at higher speeds.

Well Sinterpuck, (I like that name), I think the problem is built in when you make a swap like that. The swap is OK, sure, but just like on my 240D after I put in the 617 turbo it rattles like heck at max torque. You get the same problem in a light single engine aircraft with wide open throttle and at max torque. Just not a solid enough body to absorb the power pulse when that engine is firing on all 5 or 6 cylinders. I think there are two things you can do. One, live with it and try to get through those noser zones as quickly as possible, and bring your shift speed up a bit. I don't think that it will cause your mpg to suffer. Secondly, do a bit of home style insulation, especially above that downpipe right behind the turbo and just below the floor of the right side front seat. Mine buzzes also, and many mention it here in NA where we are converting many 300D/Sd to the four speeds that came here in 240Ds That buzz is probably why MB didn't send over any 300D manual shifts. If you have an automatic the torque convertor will eat those vibes as fast as the engine can make them, but the clutch and manny tranny has no appetite for such stuff. Insulate it out and make sure when you insulate that you don't build a water trap that will cause rust and eat you floors and doors.

If you take a passenger along they may be able to move around the cabin more and get a better idea of where the noise is coming from.

Also you you compound that problem with the 5 speed. I have to work against it in just 3rd and 4th gear, you get it in 3rd, 4th and 5th. You say it is "undriveable". Hard to believe, but if you are confined to driving in some hilly areas and can't hardly every get past the buzz zone, well that is difficult. But line the floors with "dynamat" which is a heavy duty foam insulating pad. Make sure the original pads and carpet are in good condition also. If those bottom plastic pads are no good, wet, rotten and broken, they will let a lot of vibes in.

I think a video might give us a better idea. I have a truck that have these lobs on the intake. I took them off and at certain RPMs I got a very annoying humming or droning sound that went right thru your body and head. I put them back on and the drone went away. It was explained by an engineer as "Helmholtz Resonance" Helmholtz resonance - Wikipedia, the free encyclopedia. something to think about


"The theory of Helmholtz resonators is used in motorcycle and car exhausts to alter the sound of the exhaust note and for differences in power delivery by adding chambers to the exhaust. Exhaust resonators are also used to reduce potentially loud and obnoxious engine noise where the dimensions are calculated so that the waves reflected by the resonator help cancel out certain frequencies of sound in the exhaust."

If it's only doing it at higher speeds I would suspect a wheel bearing or differential related issue

From the OP's description it's doing it during low speed high load conditions. I think it's important to get back to step one of finding out where the noise is coming from.

Ohh alot of suggestions here now, i like it!

@junqueyardjim: I had the the same issue with the standard engine aswell and i have been in multiple w123 with a om602 and 5speed gearbox and none of them has been this bad.
Unfortunately there is no dynamats on this world that will stop this, the whole body of the car is making a LOUD noise, it's really really bad.
My floor carpets with with the pads are ok aswell.
It is drivable, just not as a daily driver.
I only get the sound on lower speeds, the car is perfect over 50mph and the sound never comes back when driving over it.

@Skippy: I tried to have multiple people ride with me (including car mechanics) to try and locate the sound but noone has been able to determine where the sound comes from, the whole body is making noise so it's incredibly hard to say where it comes from.

@Drago: Here you have a video, the sound that you can hear is the noise the car makes, unfortunately the sound is not to good.
I have one of those in my intake and it's there.

Do note that i have an kmh speedo and not a mph one so you dont get confused =)
https://www.youtube.com/watch?v=wvZpYc7U7Jg

@sassparilla_kid: It's only lower speeds and all the wheel bearings is ok as stated in the first post.
The diff is a bit worn, the pinion bearings is just fine but there is play between the pinion gear and the crown wheel but just not as bad as they usually are.
And all the worn diffs making sounds i've experienced so far has made a whining noise, this is a very deep noise.