Hello All. Could use some help diagnosing problem.

While driving between 50-100 km/hr I'm getting an intermittent 'humming' sound. This occurs when the accelerator is lightly engaged, sound goes away when I stop accelerating (ease off the gas and coasting) or when I speed up quickly. Sound appears to be coming from underneath the floor boards.

Mercedes dealership diagnosed problem to be bearing (hanger bearing) on the drive shaft. When dealership was diagnosing problem they found exhaust support on back of trans was loose, support was removed and sound still there. Also they thought noise coming from under panel on passenger side of fuel tank, insulated tank bracket, noise still there. Road tested with 2 techs from Mercedes dealership. Therefore they diagnosed it as hanger bearing.

But after bearing and hanger was changed and noise still there, it is not the cause of the noise.

Upon further investigation myself I find that when car is in 4th gear around 60-80km/hr and tach is around 2200 rpm sound is more frequent? When I immediately switch gear to 5th, sound goes away, or again when I accelerate or ease off the gas(coasting). When car is in 5th gear sound is quite loud when around 100km and tach 2200 rpm, again sound goes away when I down shift to 4th or accelerate/ease off.

Suggestions???

Much thanks.

 

I should also mention that the tires are new, less than 2000km. GoodYear Assurance Tripletred.

On weekend, had car on hoist and put car in 4th and 5th gear (drive), could not duplicate sound. Therefore sound only when there is load on vehicle.

Went to the MB dealership this morning, service guy suggested swap all 4 tires with new set, and change transmission fluid. Possibly also differential fluid. Will try that these this week myself, except for trans fluid change. Booked appointment with dealership for trans fluid and further diagnostic (if humming continues).

I'm perlexed?

 

Well I changed the 4 tires with a different set (winters) as well as the rims. Went for a test drive and humming sound still there. So it is not the tires or rims.

I don't know if this has any impact or if I was just imagining it, but weather was raining this evening, so I switched the transmission setting from Standard (S) mode to Winter/Wet (W) mode. I found that the humming sound could possibly be less in W mode? Again because of the intermittent nature of the humming I'm not 100% sure if W or S has any impact. Looking at the manual, the W and S modes differ in that at standstill the S starts in 1st gear, whereas in W mode standstill starts in 2nd gear. Another difference is the ratio of power transmission is also dependent on the mode selected. One thing that I did notice for certain was when the humming began in S, if I changed the mode to W, the humming would stop for momentarily; although there still was humming present in D (5th gear). As mentioned in my previous post, the same is the case in that the humming would stop if I eased off the accelerator or accelerated quickly.

Again humming begins around 1500 RPM @ aprox between 50-60 Km/hr, then again around 1800-2200 RPM 80-100 km/hr.

I'll be changing the differential fluid next to see if that will solve the humming problem, but any ideas would be appreciated. Thanks

 

I don't think changing your diff fliud or transmission fliud will solve the problem. The problem with noises like that is that they are soooooo hard to find sometimes. Check your exhaust and see if there is any loose heat shields on or around the exhaust. Also, make sure that your under covers are not loose or that there is nothing loose under the vehicle. It may be speed related depending on wind while you drive.