rumble

[cosytoes]

I have a 2002 rav4 d4 that has an intermittent problem. when travelling at speed or just normal driving there is a loud rumble from underneath which causes the whole floor pan to vibrate, this disapears just as quickly as it happens.Just recently it seems to be happening more often than it used too,i took it to the garage who have checked the wheel bearings and prop shaft but cannot see any obvious problem.i am hoping someone on here may have experienced the same problem and can point me in the right direction as too what the problem may be.Tia mark