Hello
My friend had the same problem. He was told it was coming from debris built up between the tyre and the wheel. He had the tyres taken off and the inner rims of the wheels cleaned the tyres back on.
This cured one but the other still leaked...
He got some of that stuff(SORRY CANT REMEMBER MAKE) you spray in through the valve that is suppoed to work and plug up punched tyres. It stays liquid inside till it finds a hole then blocks it.
Anyway he thought it was coming though the wheel itself and this stuff did the trick...
He bought his from the greatness that is HALFORDS

He wont learn, bloody BMW drivers
Hope this helps maybe worth a go for about £10
Mark