I dont know about the whirring noise (i noticed the 307 1.6 HDI does it to tho) i think it may be some kinda purge valve but if its the same noise i think it is it's normal. but i may be able to help with the others.
With the radio i personally would just replace the head unit. you should be able to pick up a surround from a place like halfords, ebay etc and shouldnt cost more than £20.
The fuel guage is not measured by the normal float as you would expect but estimated by the computer. This is the reason why a small amount of fuel put in to the car ie 5 litres may not register on the gauge. by parking on a hill you confuse the computer in to thinking that it has less/more fuel than it actually does. If in doubt park on the flat and switch off ignition for a few minutes and start up again.
With your oil level i personally would check it manaully as 3" aint much of an angle. although i doubt its below the minimum mark when on a flat surface.
Craig