You didn't give the year. Exhaust resonance is my guess. Maybe resonance
accentuated by rusting thin exhaust
pipes. Tires are a maybe as is a wheel bearing or u-joint, but those aren't
usually specific to that speed and
have other signs as well.
| I have a Chevy Suburban R2500, 2WD, 454.
| When I am traveling on the freeway, at approximately 75-80 mph, the engine
| begins to give off a pulsating moaning sound. If I reduce the RPM's, the
| noise disappears. There is no vibration felt. Could this be the harmonic
| balancer? If not, any ideas?