I've recently bought a Peugeot 307 se 1.6 petrol 55 plate and its making a slight whining noise from gearbox in high end of 4th gear and 5th gear, gets a little more noisy under throttle and throttle off. Worse when driven for longer periods of time. Is this the start of the gearbox failing? Should I worry or is this a common fault?

You most likely have two problems. 1. The throw out bearing is probably bad, and 2. This has caused some gear shearing within the gear box. Best fix is to have the gear box removed, cleaned, check for chipped, cracked gear teeth, and replace with good gears. Also, check the synchronizer , make sure it is in good working order.

It appears u do not reach the speed it is required for the 5th gear. Plz check it otherwise there should not problem exclusive with the 5th gear

sounds like the gearbox selector, by the way have you checked the gearbox oil to see if its low or contaminated

this would be an internal bearing inside the gearbox on the input shaft......you have the shft coming in form the engine, and the input shaft houses 1,2, 3rd and 4th gears.....the 5th gear is located on an intermediate shaft.....and then you have the output shaft...now if you had said all gears i would have said output shaft bearing......and if you had said 5th gear only, then this would be the intermedaite gear.....

but you are only saying 1,2 and 3......why its not happening in fourth is that the gear ratios that the shafts turn in correlation to thre gearing changes as you get to 4, like on a 10 speed bicycle but in 4th it is a 1:1 ratio, so no excessive loads on the shafts are noticeable....

This will require a transmission overhaul......

i can't change into 2nd,4th and reverse gears. i think its a gear lever cable not sure doe any ideas?

do get like a clunking sound shifting between first an reverse as you let the clutch out?.if so its probably a universal joint either the one up by the tranny or the one by the rear axle.

This would be most likely the first noise of a wearing input shaft bearing. It will only get louder, and eventual failure of the bearing will probably mean catastrophic gearbox failure. It stops when you put your foot on the clutch because the gearbox stops spinning then, and it is quiet in 4th and 5th because there is no side loading (as there is in 1st, 2nd, and 3rd) on the failing bearing. It wouldn't matter what sort of car it was, the gearbox should be overhauled as soon as possible.

No good news on this one I think
Your whining sound problem is the Layshaft bearings in the gearbox are badly worn. It also seems that the baulk rings are damaged on 5th gear.
John

sounds like syncro bearings in trans

sometimes a nut comes loose behind the end cover on ml5 box.