4th gear grinds when changing down

[Johnmiles2121]

I have a Subaru GX Sport Estate 2003 and when I change down to fourth gear it makes a grinding noise. It will only grind when moving down to fourth gear and I am wondering if this could be a problem with the gear box. The rest of the gears work fine it is only when I am approaching a big hill and need more power and decide to change down then I hear the grinding. 

Any help will be much appreciated. 

Thanks 🙂

[Siluro]

Maybe drain the oil from gearbox and check the magnetic plug for large metal parts and the oil. You will probably find some tiny metal on the plug that smears like a paste this is probably normal. But anything larger may give you a clue. New oil may solve it.

[Johnmiles2121]

Thank you for the help!