A good way to determine if it is engine noise or otherwise would be to carefully shift to neutral and let the engine idle while you're above seventy. However, I wouldn't put it back into drive at that speed, I'd coast to a near stop before putting it back into drive, just to be on the safe side.
If the noise goes away it's in the engine. If the noise is elsewhere, you'll still hear it, and without the engine noise, it might even be more apparent where it's coming from.
'84 Euro 240D, 516K miles
'88 300E 5 Speed
'81 300D Daughter's Car
Over 800,000 miles in