Strange Groaning Noise on Acceleration

[Bodeka]

This is a really weird one.

Whenever the car has been driven on the motorway for anywhere from 20 minutes to an hour, it starts to groan on light acceleration. If I put my foot on the gas, it groans. If i keep it steady, it doesn't groan. If I take my foot off, it groans as the pedal resets to the rest position. Sometimes it groans without touching the gas too.

This ONLY ever happens after that initial period, quick trips on the motorway do fuck all. It doesn't matter what revs I'm doing. When I came off the motorway and was at a traffic light today, I gave it a few revs and it didn't come back. Then as I pulled off the motorway and was doing normal speeds, the noise had completely gone.

This is driving me loopy and I'm not the best with cars, what could it be?