Im getting a bit of a whine noise off the forester, it sits for weeks on end not used which i suppose isn't good for it, the other day i tried to start it but the battery went flat, so i rolled it down the hill and it started, although now its making a grumbly whine noise from underneath:

 

Any idea of what this could be, any chance I've damaged the gearbox or differential?

Sounds a bit like the bigends to me mate :( 

The big end bearings? 

But this noise doesn't seem to be there when the car is stationary, I'm thinking its something in the brakes or transmission.

I couldn't really tell that much from the video to be honest I had to turn everything up full just about volume wise but the parts I could hear sounded like it. Might be a timing belt tensioner thats gone bad should be able to hear it better from under it .   

If it's been stood for a while it may be just rust on the discs, they all need driving.

Big ends don't whine, they knock, clatter and bang!

If it does it all the time the vehicle is moving then most likely a wheel bearings, if it only does it under power then more likely to be a diff.

I couldn't hear much from the video so was guessing :) 

Its really hard to tell anything from that video, I played it through my ipad, then out through the surround sound speakers - I think you have too many empty crisp packets in the car :-D

 

Initially it sounded indeed like a tensioner/belt whine, but that should be evident when stationary as well obviously. Its unlikely to be the diff making noises unless the unit is bereft of oil and the condensation from leaving it standing has caused any surface rust, but that really is outside the chances.

 

I would suggest its something more to do with the drive train connections, and most likely from the central prop shaft joint area, which is more exposed to the elements than anything else. Just keep a check on it as if it seizes it can cause all sorts of problems at both ends! I've just bought a Landrover Freeloader with a knackered IRD from a similar issue.