"Buzzing" Engine?

My '99 recently started to emit a buzzing noise when driving. It seems to start at about 50km/h. The noise dissipates as I slow down, and can't be heard at all when idling. I thought maybe it was the 4WD, so, as I drove, I cycled through all four modes, and it didn't seem to correct anything. I cant even really pin-point where it is coming from - but, its gotta be somewhere under the hood. Any ideas? Could it be a spark-plug problem?

Could be your tyres. Check the pressures. Make sure there are no stone chips stuck in the tread.