Got a 2000 Subaru Legacy GT Limited. Have a low frequency hum coming from the rear that starts at about 45mph and goes away at 55. I've replaced one rear tire (one is brand new, other is very good), both rear wheel bearings/hubs, both rear cv half shafts, and rear control arm bushings. It was clunking when shifting and going into gears (auto trans), and I saw the differential mount bushings were separated. So I put in half a split bushing in each. Still clunks in reverse but not in forward. But none of these repairs have made the low frequency hum noise in the rear change. Could it be the driveshaft u joint? I don't get that rattle that I hear on youtube videos. It has 277,000 miles. It was a North Carolina car and it looks new under the hood and undercarriage.

all 4 tires the same brand/model/size?  any bucking/jeking/grabbing in sharp turns maneuvering in parking lots? will the car do tight circles on dry pavement at or 'just' above idle rpm?

 

try swapping tire pairs front to back, inspect suff like lug nuts and brakes when you do this off course.

 

you try the FWD fuse too.

A mildy bad wheel bearing can get swamped by road noise as you speed up. Goes around once for every wheel revolution.

 

A wheel bearing will change in pitch at the same rate you speed up, a driveshaft component will 'change pitch' about 4x as fast as the speed change. Goes around about 4 times for every wheel revolution.

all 4 tires the same brand/model/size?  any bucking/jeking/grabbing in sharp turns maneuvering in parking lots? will the car do tight circles on dry pavement at or 'just' above idle rpm?

 

try swapping tire pairs front to back, inspect suff like lug nuts and brakes when you do this off course.

 

you try the FWD fuse too.

 

Tires are the same. I haven't noticed any bucking or drag during tight turns, but I know what you mean (my 90 Legacy used to do that). I'll give the FWD fuse a try. Front looked ok, I changed shocks, ball joints, inner/outer tie rod and front axles and didn't see any issues with the brakes, bearings or hubs. Noise seems to be coming from the rear, though maybe I'm wrong.

 

CNY_Dave, what would you say it indicates that the noise only seems to happen from 45 to 55? I was thinking driveshaft since the rear diff mount bushings were bad and it was moving up and down somewhat. I'm guessing the diff housing would go up when in reverse and bounce off the bottom when shifting going forward. Not sure what that would damage.

Iirc I had a car do something like that once. Turned out to be the driveshaft. U joints were shot. Because it spins fast, a little extra stiffness or slop can make it off balance, and go in and out of resonance in a narrow speed range.

I'm thinking that's probably it DaveT

Agree one of several things tyres, wheel bearings, drive shaft universal joints / carrier bearing, cvs / axles (the replacements could be faulty) , transfer clutch etc, bearing going in rear diff (think GTs had LSD rear diff) , exhaust vibrating / resonating against something , worn rubber mounts somewhere.

 

Agree the clunking could be the driveshaft / rear diff.

Noises can travel and fool you so it pays to investigate all possibilities.

 

ps had an old RWD car that used to resonate at 55mph will no passengers in back seat but used to go away when passengers in back Tracked it down to worn lower rubber shockie mounts. And exhaust vibrating / resonating against something on another occasion.

Edited December 3, 20178 yr by subnz

Tires are the same. I haven't noticed any bucking or drag during tight turns, but I know what you mean (my 90 Legacy used to do that). I'll give the FWD fuse a try. Front looked ok, I changed shocks, ball joints, inner/outer tie rod and front axles and didn't see any issues with the brakes, bearings or hubs. Noise seems to be coming from the rear, though maybe I'm wrong.

 

CNY_Dave, what would you say it indicates that the noise only seems to happen from 45 to 55? I was thinking driveshaft since the rear diff mount bushings were bad and it was moving up and down somewhat. I'm guessing the diff housing would go up when in reverse and bounce off the bottom when shifting going forward. Not sure what that would damage.

 

Sometimes it does keep making noise as you speed up but road noise drowns it out.