Thanks for all the input.
Having rechecked the tach, I note 60mph=3100 rpm and 70mph=3500 rpm. Figures seem to be in line with those mentioned by the senior members of this board.
The noise I mentioned may be more akin to loud fan noise emanating from front of engine area. It comes on under load at around 40 mph and gradually gets noiser as speeds increase. Any ideas on this issue.