Get it up to speed (whatever is prudent), pull in the clutch and coast a bit.
Rumbly?
Put the rear brake on.
No rumbly?
In all likelihood, bearings as the poster above me theorized. Applying the brake allows the wheel to move around the bearing axis and eliminate any slop which has developed as a result of bearing failure.
The exact same scenario manifested itself earlier in the year with my bicycling buddy's front wheel. Her bike was bought new last July and had maybe 250mi on it since then. On our first ride this season the front wheel began getting loose and noisy. Brake application (disc) quieted things down.
Upon teardown I found a scored cone race, and while the hub (cup) race itself didn't look bad it wouldn't stay quiet - even after new cones, bearing balls and grease. Solution was another hub.
Much easier to change out our cartridge bearings.
