.....when i'm on the interstate, or on a long downhill and i step on the brakes i get a fairly loud "rumbling" noise.
no pedal pulse, no squeel, no bumping or clunking and it stops as soon as i release the brake pedal.
in town everything is normal and the brakes work fine, no noise.
i will check the rotor runout, but i'd think there would be pulsing at the pedal.
i'd think bad wheel bearings would always make noise.
maybe the axles/cv joints??
any ideas?
thanks

 

Check whether the brake pads are worn down. I'd probably call that a "grinding" sound, but checking the pads is very easy if you're planning on checking the rotors.

What most people call "warped" rotors, is usually brake pad material unevenly transferred to the rotors, producing areas of the rotor that have different amounts of friction. But usually you feel that as the car shuddering when you apply the brakes.

Check all the suspension joints. Maybe a loose ball joint can cause this, and the reason you don't hear it around town is the braking forces are smaller.

 

Thanks Jim, good idea - i will check those Akebono pads asap
Cheers

 

Hi Jim, yes, i checked the runout & pads & balljoints today. Runout seems like 0mm (or close to that), balljoints stable, but pads about 1/16" or less.
And they only have 51,000 miles on them - HA! The wife was driving up to & DOWN from the local ski area almost daily this past winter
and that may have accelerated the wear, whatever. New Akebono pads are now ordered. I just have to assume when pads that thin get hot
they make noise/vibration, or - as you suggest - they may be more susceptible to the effect of build up on the rotors. We'll see, but it's got to be a good start.

Thanks! .

 

Get the rotors resurfaced...?
The uneven transfer of pad material is sometimes visible as a different sheen or texture on the surface of the rotor. But sometimes it's subtle & difficult to see.