'91 B2600i ext cab with 164K miles. I have an annoying, not to noisey, buy loud enough to bother me, roar in my drivetrain at about 60 mph. I have jacked the vehicle up and put my ear via a tool to the drive shaft carrier bearing, pinion bearing and axle bearings which have all never been replaced (did replace the u-joints not too long ago thinking this was it). All the checks seem about the same noise level, not obvious bearing noise found. I just put on a new set of tires as well to no avail. Has anyone else experienced this or found a cure? thanks in advance

 

First thing I always think of when I hear the word 'roar' when relating to Mazda rear wheel drive is a seized cooling fan clutch or bent/broken/deformed fan blades.

 

First thing I think of when the roar or whine in the drivetrain complaint came in was a ring and pinion in the rear end. The magic words on the RO was 60 MPH.

 

We'll, not a load roar, but more of a "you know something ain't right" faint roar. I'm going to go with the ring and pinion, after double checking with a more solid iron to ear, it seems to be a little noiseeeeey. I know what you mean with the cooling fan, got so hot here in TX one year I opened up my fan clutch, drilled and installed two bolts inside to 'seize' it up and keep the thing cool. Wife would come home some days and wonder why it whired loader sometimes then others on the highway, then i showed what the button on the shift lever does......O/D off

 

Has the truck been lifted or lowered?? Many times ppl do one of those and don't correct the pinion angle and it will give them a 50-60 mph roar or vibration.

Getting the ring and pinion out is quite a chore and would probably be the last thing I checked, have you changed the fluids in the diff and transmission? How do (did) the fluids look? Do you get any backlash in the driveshaft when you try to turn it on the ground?

What kind of tires did you put on it?

Also if someone brinelled one of the wheel bearing races during replacement that would cause a similar symptom depending on the severity.

 

This might be way off, but I have a growling sound when my belts are a certain tension. The Alternator belt will get into a harmonic growling sound right at 60, so I have to either loosen or tighten it and it will change the speed that it happens. It had nothing to do with drive train, but I could actually feel it as well as hear it. Rev the engine up to highway RPM with the hood open and see if the alternator belt is still smooth. Just a thought.