transmission noise?

on a couple of occasions I have heard a funny noise from my transmission.

It has always happened when I have accl up through the gears to about 200kph+ (on a private test track of course) then let off until about 100 - 120 kph & cruise. I get a real coffee grinder kind of rumble. If I cruise carefully for a bit & give it throttle a couple of times the noise goes away.

This has only happened 3 times & it was all on the same journey when I 1st got the car. Have not heard since but then again I have not had the use of a private test track

I don't think it could be the tyres even tho one of the rears needs changing.

any ideas?

BTW I have not checked the gearbox oil