[ROVERNET - UK] noise from underneath

[henning helmer]

I have a noise problem with my 2000SC from 1967, 28.000 genuine 
miles.When driving along at 40-50 mph
there is a howling sound slightly varying in note (with a frequency of 
appr.3-5 per second).
Of course diff oil has been controlled and the splines lubricated. The 
noise is there whether engine pulling or on the
overrun.
Nor is there a change in the note when swinging the car from side to 
side on the road.
Concerning the gearbox mountings, should it be necessary to replace 
after only 28.000 miles?
My local mechanic recommend me to drive on and see what happens!!!
Does anyone have experienced similar problem? As you can understand, 
the car is in very healthy condition.
regards

henning helmer
tel. +45 49192700
fax +45 49190661