when I put the car in 5th gear there is a slight whistle / whine... does not do in other gears.

 

If the noise only happens as you put the car in 5th gear I would say a worn 5th gear dog. If the whine is fairly constant and gets louder with hard acceleration or aggressive engine braking I would suspect a worn input/output 5th gear.

Either way I would recommend a transmission oil change to remove any metal that might be floating around in the case. Aside from that, take it easy on 5th gear. Don't shift hard if the dog is going out. If the input/output pair is whining, it might drive for years like that or it might go into full scale gear chewing the next time you drive it. It's impossible to say.

Being a BMW, its hard to find shops willing to do transmission rebuilds. You may yet be better off buying a rebuilt or used transmission and selling your current transmission as a core before it goes into full failure. Alternatively, just wait 'til it goes boom, it could last a long time.

Any way you slice it, best of luck!

 

You have a Midland transmission in your car, and they were notorious for gear whine as they aged, followed by self destruction. Not to say it's going to happen in the next months, but the Midland doesn't have a very lustrous track record.