Clonking sound when pedalling in 5th and 6th gear?

I just purchased a bike, with 7 gears. When I cycle in 5th and 6th gear it’s making a clonking noise when pedalling. It doesn’t do this noise in any other gear just the 5th and 6th.

My partner said it sound like it’s coming from the crank shaft but it’s not that. It is from the back somewhere. He has checked it and it’s not misaligned and he has tightened up the screws if it was the derailleur why would it only happen in 5th and 6th gear. I’m confused by it.

I would anyone know what the cause of this is and is it a fault with the bike?