Irregular droning noise when under load and higher revs

This is going to sound funny but here goes.... Do you know that sound a ruler makes when you put it by the edge of the desk, hold it, bend it down and release it and let it wobble up and down in speed?

It happens when accelerating hard in 4th , 5th or 6th... Slightly in 3rd also but not that much. particularly uphill - so I'm thinking it happens when the load is high...

The car has developed this noise for a while now.... It never got worse, it's been like this for almost a year now. I tried recording it but a phone won't pick that sound up. Some mechanics suggested plastic parts might be vibrating but I doubt it's plastic.... Tried driving with and without the engine cover or bottom engine cover.... no difference.

It has no relation to revs other than it occurs approx between 2.5 and 3.5 thousand. The sound does not increase with the revs, its just randomly happening within that rev range... Sometimes I have the impression that something will snap off somewhere....

The engine has power and pulls like a train... no issues there....

I wish I could somehow  record inside the engine area while driving to hear it more clearly... Any ideas? Could it be the aux belt tensioner?  

Standard BKD engine driven 260.000 km.