My Logic7 (w/o iDrive) speakers just started doing this too. I usually have to give the door a bump for the sound to start up again. I'm thinking it might be a loose cable or something but it happens to both the tweeter and door speaker at the same time. Weird.
'07 E90 328i / Sparkling Graphite / Black and Aluminum Interior / 6MT / ZSP / Xenon / Logic7 / BMW Performance Exhaust / BMW Performance Short Shift Kit / Modified CDV / Euro Light Switch / Dæos DRL Harness / V1 Fuse Box Hardwire / Blacklines / Darmstadt Military Sales