should the aluminium dipstick have a gasket?Mine does not, but I do not think the leak comes from there.
The dipstick seal is under crankcase pressure and without a proper seal it could certainly leak and interfere with the correct operation of the engine breather which is designed to pump down the crankcase to a partial vacuum. An air leak at any point into the crankcase will cause problems and besides the dipstick seal that includes the rear main seal and front crank seal. So even if you don't hear the noise from the dipstick lack of the correct seal could still be the primary cause of your problem.
Of course the noise could still be something else but either way your dipstick needs a good air tight seal.
I think there should be a gasket on the early metal dipsticks and an O ring seal on the later dipsticks which may be plastic. In practice the O ring works on both types. I would think there is no need for a BMW specific part. A generic Nitrile rubber O ring of the correct dimensions will work fine. If you need the O ring dimensions I'd be happy to measure mine.
