A whirring or whisper type noise has developed in our '95 Explorer. It it most noticeable when coasting at speeds of 40mph or less. At
40mph it occurs approx 2times per second and slows as the car slows to about 1 per second at very slow speed. It is a "soft" sound, low frequency. Checks by our regular mechanic lead to some work by another shop. After checking both differentials, and all wheels & bearings, the 2nd shop diagnosed two bearings that were failing in the transfer case. This did not fix the noise problem, although it may have caught a problem that was just beginning to appear. The noise we are hearing does not appear to be affecting the way the Explorer performs, but I don't want to continue to operate it until we have a breakdown. Any ideas as to what the root problem is?Some further background - vehicle has 110,000 miles, we are the 2nd owner, having bought it "off-lease" in late '97 with 25,000 miles on it. We have been careful to maintain it per Ford's maintenance schedule. Unplanned service includes replacement of the front left hub at 39,000 miles. It is rarely driven in 4wd.