About a year ago I moved to east Africa and bought a 1983 Range Rover Classic, a wonderful beast of a car. It is 4WD, manual transmission and has a V-8 gas (petrol) engine. As I understand, she had been used very little for many years, but with the help of very clever young bush mechanic it is finally coming back to life.
There is one (ok, 2) problems which we cannot seem to fix: Around 35 mph, the car starts to make a reverberating hum sound: “mmmmm… mmmmmm… mmmmmm”. The sound is reasonably loud and seems to resonate over a period or 2 or 3 seconds. At faster speeds the humming quiets down just a bit–of course maybe it is just being drowned out by the roar of the ancient engine! The sound seems to come from somewhere under the car (not under the hood), and happens even when clutch is in–so I don’t think this could be an engine issue. Could it be the differential? Something else?
We’ve tried wheel balancing but that doesn’t seem to do anything. Tires are at half-life and look fine. Any ideas? What can we do about this? No good garages here but my mechanic is impressively methodical and we have a copy of Haynes to reference.
I should probably also mention that last spring a rush of water on our street pushed the car into a flooded ditch. With the pushing and lifting of about 15 strong guys were were eventually able to pull it out, but afterwords the car mostly sat for a few months and by last month when it was reactivated, it had developed a few small problems as a result (frozen parking break, inconsistent starter, etc.). These have been fixed and I don’t have any particular reason to believe that the flooding is the cause of the humming, other than that I had not noticed the problem before.
The other problem (which I don’t imagine is connected but thought I should mention just in case) is a minor rattling (of the clutch?) when the car is in neutral and also in forth gear, and a bit of slipping in first and reverse which we can’t quite get our minds around–engine revs up uncontrollably, embarrassing when i pull into a parking lot. My mechanic tells me the clutch/clutch plate is not overly worn.
Your “mmmmmmm” sound could easily be a wheel bearing, differential or transfer case. Get it up on a lift and listen for where there sound is coming from.