mystery sound

hey all. i know the "what's this sound?!?" question is maybe annoying and definitely subjective, but it's worth a shot. '96 2.7L 4wd manual transmission, ~235,000 miles.

- it's rotational, i hear it most clearly between 25-55mph, it sounds like a low bass-tone whoosh, kind of like an old locomotove engine chugging.
- there's no change between gears or in neutral
- no difference when driving in H4
- my brakes feel fine, pads new within 5000 miles, no intense vibrations like i'd expect from warped rotors
- it's hard to pinpoint the location but seems like its coming from the center of the vehicle below me.

any obvious things to look for and rule out? i'm worried that somethings wrong with the transmission, but i haven't encountered that before.

thanks for the help.

 

At a stop engine running in N or in gear

If u press clutch pedal down to floor does sound go away?

 

it's rotational and correlated with speed (increased tempo), so there is no sound at a stop. there is no change based on gears/neutral.

 

Bent brake backing plate, maybe.

 

Common things to check would be carrier support bearing, slide under the truck and push up and down on the driveshaft and look for excessive play. Next thing is the needle roller bearing in the front diff where the axle shaft ride, East Coast Gear Supply has a replacement bushing for that. I wish I had a better way to diagnose this... you might have to take it to a shop and have them put it on a lift and run it up to speed with someone undernearth it with a stethascope.

 

our speedo cables are noisy at one end if failing
one behind instrument cluster(but u said hear whoosh center under)
one screwed into the tranny (poss(?)

i like glamisman's cause too
lots of threads re: carrier bearing failure sounds/symptons too

IMO your can visually check (like Glasiman stated)
push up and down on drive shaft......inspect the rubber bush on the CB
likely will see cracks or missing chunks of rubber